Novel hydroxyphenylpyruvate dioxygenase polypeptides and methods of use

ABSTRACT

Novel hydroxyphenyl pyruvate dioxygenase (HPPD) polypeptides, variants and fragments thereof, as as well as polynucleotides encoding the same, capable of conferring commercial levels of conferring HPPD herbicide resistance or tolerance to plants. Compositions include amino acid sequences, and variants and fragments thereof, for HPPD polypeptides, as well as polynucleotides encoding the same. Methods for the production and use of HPPD herbicide resistant plants that express these novel HPPD polypeptides, methods for selectively controlling weeds in a field at a crop locus, and methods for the assay, characterization, identification and selection of these novel HPPDs are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 12/692,552, filed Jan. 22, 2010, which claims priority to U.S. Provisional Application No. 61/224,661, filed Jul. 10, 2009, and to U.S. Provisional Application No. 61/146,513, filed Jan. 22, 2009, each of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to novel hydroxyphenyl pyruvate dioxygenase (HPPD) polypeptides that confer herbicide resistance or tolerance to plants and the nucleic acid sequences that encode them. Methods of the invention relate to the production and use of plants that express these mutant HPPD polypeptides and that are resistant to HPPD herbicides.

BACKGROUND OF THE INVENTION

The hydroxyphenylpyruvate dioxygenases (HPPDs) are enzymes that catalyze the reaction in which para-hydroxyphenylpyruvate (HPP) is transformed into homogentisate. This reaction takes place in the presence of enzyme-bound iron (Fe²⁺) and oxygen. Herbicides that act by inhibiting HPPD are well known, and include isoxazoles, diketonitriles, triketones, and pyrazolinates (Hawkes “Hydroxyphenylpyruvate Dioxygenase (HPPD)—The Herbicide Target.” In Modern Crop Protection Compounds. Eds. Krämer and Schirmer. Weinheim, Germany: Wiley-VCH, 2007. Ch. 4.2, pp. 211-220). Inhibition of HPPD blocks the biosynthesis of plastoquinone (PQ) from tyrosine. PQ is an essential cofactor in the biosynthesis of carotenoid pigments which are essential for photoprotection of the photosynthetic centres. HPPD-inhibiting herbicides are phloem-mobile bleachers which cause the light-exposed new meristems and leaves to emerge white. In the absence of carotenoids, chlorophyll is photo-destroyed and becomes itself an agent of photo-destruction via the photo-generation of singlet oxygen.

Methods for providing plants that are tolerant to HPPD herbicides are also known. These methods have included: 1) overexpressing the HPPD enzyme so as to produce quantities of HPPD enzyme in the plant that are sufficient in relation to a given herbicide so as to have enough of the functional enzyme available for the plant to thrive despite the presence of the herbicide; and 2) mutating a particular HPPD enzyme into an enzyme that is less sensitive to inhibition by herbicides. Methods for mutating HPPD enzymes for improved HPPD herbicide tolerance have been described (see, e.g., PCT Application Nos. WO 99/24585 and WO 2009/144079), and some particular mutations of plant HPPD enzymes (e.g., mutation of G422 in the Arabidopsis HPPD sequence) are purportedly capable of providing some measure of tolerance to mesotrione and other triketone herbicides. However, the enzyme kinetic and whole plant data reported thus far are insufficient to conclude whether the reported mutational changes confer commercially significant benefits over the corresponding wild type enzyme(s).

Furthermore, while a particular HPPD enzyme may provide a useful level of tolerance to some HPPD-inhibitor herbicides, the same HPPD may be quite inadequate to provide commercial levels of tolerance to a different, more desirable HPPD-inhibitor herbicide (See, e.g., U.S. Patent Application Publication No. 20040058427; PCT Publication Nos. WO 98/20144 and WO 02/46387; see also U.S. Patent Application Publication No. 20050246800 relating to the identification and labelling of soybean varieties as being relatively HPPD tolerant). Moreover, mutated versions of HPPDs from cool-climate grasses with improved resistance to triketone-type herbicides have yet to be reported. Such mutants would be highly desirable, as HPPDs from cool-climate grasses are preferable to other types (see, e.g., PCT Application No. WO 02/46387 and Hawkes et al. 2001 in Proc. Brit. Crop Prot. Conf. Weeds 2, 563). Accordingly, new methods and compositions for conferring commercial levels of HPPD herbicide tolerance upon various crops and crop varieties are needed.

BRIEF SUMMARY OF THE INVENTION

Compositions and methods for conferring hydroxyphenyl pyruvate dioxygenase (HPPD) herbicide resistance or tolerance to plants are provided. The compositions include nucleotide and amino acid sequences for HPPD polypeptides. In certain embodiments, the polypeptides of the invention are novel HPPDs that derive from plants and that confer resistance or tolerance when expressed heterologously in other plants to certain classes of herbicides that inhibit HPPD. In particular embodiments, these HPPDs comprise amino acid sequences set forth in SEQ ID NOs: 2-12, particularly SEQ ID NOs: 2-8, and polypeptides having at least about 99, 98, 97, 96, 95, 94, 93, 92, 91 or 90% sequence identity to SEQ ID NOs: 2-12 that exhibit HPPD enzyme activity.

Preferred novel HPPDs are likewise those that, in comparison with HPPD enzymes of the prior art, exhibit superior tolerance to one or more types of HPPD herbicide and where tolerance is characterised in vitro by the numerical value of the parameter (k_(off)×k_(cat)/K_(m HPP)) and where k_(off) is the rate constant governing the dissociation rate of the complex of the HPPD enzyme with herbicide and k_(cat)/K_(m HPP) is the the catalytic turnover number divided by the K_(m) value for the substrate HPP (4-hydroxyphenyl pyruvate).

In a further embodiment of the current invention there is therefore also provided an in vitro method for characterising and selecting HPPDs that confer superior levels of tolerance to HPPD herbicides based on measuring and comparing values of k_(cat)/K_(m) HPP and k_(off) or functional equivalents of these parameters.

In further embodiments the polypeptides of the invention are catalytically active mutant HPPDs that derive from plants and that, relative to the like unmutated enzyme, confer superior levels of resistance or tolerance to certain classes of herbicides that inhibit HPPD. In particular embodiments these mutant HPPD polypeptides comprise one or more amino acid sequences selected from SEQ ID NOs: 15-19, wherein SEQ ID NOs: 15-19 have one or more amino acid substitutions described as follows:

With respect to the sequence (L,I,R)(V,A)(G,A)DVL(S,T) (SEQ ID NO: 15), the first L, the I, or the R is replaced with any other amino acid, particularly E, D, G, C, N, Q, S, and A, and more particularly E, C, A and D.

With respect to the sequence G(I,V)LVD(R,K) (SEQ ID NO: 16), the L is replaced with any other amino acid, particularly M, F, Y, I, A, W, and V, and more particularly M.

With respect to the sequence DH(V, I, M)VGN (SEQ ID NO: 17), the first V, the I or the M is replaced with any other amino acid, particularly L, A and I, and more particularly L and I.

With respect to the sequence GGF(E,D)F(M,L)(A,P) (SEQ ID NO: 18), the A or the P is replaced with any other amino acid, particularly R, K, H, N, I, L, T, S and Q, and more particularly R, I, L, H and K.

With respect to the sequence CGGFGKGN (SEQ ID NO: 19), the second G or the K is replaced with any other amino acid. In certain embodiments, the second G is replaced with an R, K, H, E, D, N, Q, A, S, T, and more particularly R, S, T, H and K. In other embodiments, the K is replaced with an S and T, and more particularly a T.

In some embodiments the polypeptides are singly, doubly, triply, quadruply, quintuply or sextuply mutant HPPDs that combine more than one of the above mutations in the various permutations (for example, 2+3, 2+4, 2+1; 3+4, 3+1; 4+1; 3+4+1, 2+3+4, 2+4+1, 2+3+1; 2+3+4+1; 1+2+3+4+5 etc.).

In further embodiments the mutant HPPD is derived from a monocot plant and, in particular, a cool climate grass species such as wheat, barley, oats or rye. In particular embodiments the mutant HPPD is derived from Lolium, Avena, Poa, Alopecurus or Sorghum species and, more particularly, is derived from one or more of the HPPD polypeptides of SEQ ID NOs: 1-8.

Exemplary HPPD polypeptides and mutant HPPD polypeptides according to the invention correspond to the amino acid sequences set forth in SEQ ID NOs: 2-8, 20-41, 49 and 50 and variants and fragments thereof. Nucleic acid molecules comprising polynucleotide sequences that encode these particular mutant HPPD polypeptides of the invention are further provided, e.g., SEQ ID NOs: 53-82, 84 and 85. Compositions also include expression cassettes comprising a promoter operably linked to a nucleotide sequence that encodes an HPPD polypeptide of the invention, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits. Transformed plants, plant cells, and seeds comprising an expression cassette of the invention are further provided.

The compositions of the invention are useful in methods directed to conferring herbicide resistance or tolerance to plants, particularly resistance or tolerance to certain classes of herbicides that inhibit HPPD. In particular embodiments, the methods comprise introducing into a plant at least one expression cassette comprising a promoter operably linked to a nucleotide sequence that encodes an HPPD polypeptide of the invention. As a result, the HPPD polypeptide is expressed in the plant, and since the HPPD is selected on the basis that it is less sensitive to HPPD-inhibiting herbicides, this leads to the plant exhibiting substantially improved resistance or tolerance to HPPD-inhibiting herbicides.

Methods of the present invention also comprise selectively controlling weeds in a field at a crop locus. In one embodiment, such methods involve over-the-top pre- or postemergence application of weed-controlling amounts of HPPD herbicides in a field at a crop locus that contains plants expressing the HPPD polypeptides of the invention. In other embodiments, methods are also provided for the assay, characterization, identification, and selection of the HPPDs of the current invention.

BRIEF SUMMARY OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts data from K_(m) and V_(max) determinations of the Avena-derived HPPD polypeptide corresponding to the amino acid sequence set forth in SEQ ID NO: 1.

FIG. 2 depicts data from inhibitor exchange experiments at ice temperature to determine the off rate (k_(off)) values governing dissociation of the complex of structure B (mesotrione) with the HPPD polypeptide corresponding to A) the amino acid sequence set forth in SEQ ID NO: 1 and B) the amino acid sequence set forth in SEQ ID NO: 41.

FIG. 3 shows a representation of binary vector 17900 for soybean transformation, conferring HPPD resistance with a dicot codon optimized oat HPPD gene encoding SEQ ID NO: 49. This binary vector also contains double PAT selectable markers for glufosinate selection.

FIG. 4 shows a representation of binary vector 17901 for soybean transformation conferring HPPD resistance with a dicot codon optimized oat HPPD gene encoding SEQ ID NO: 50 and also conferring tolerance to glyphosate (selectable marker).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides compositions and methods directed to conferring hydroxyphenyl pyruvate dioxygenase (HPPD) herbicide resistance or tolerance to plants. Compositions include amino acid sequences for native and mutant HPPD polypeptides having HPPD enzymatic activity, and variants and fragments thereof. Nucleic acids that encode the mutant HPPD polypeptides of the invention are also provided. Methods for conferring herbicide resistance or tolerance to plants, particularly resistance or tolerance to certain classes of herbicides that inhibit HPPD, are further provided. Methods are also provided for selectively controlling weeds in a field at a crop locus and for the assay, characterization, identification and selection of the mutant HPPDs of the current invention that provide herbicide tolerance.

Within the context of the present invention the terms hydroxy phenyl pyruvate dioxygenase (HPPD), 4-hydroxy phenyl pyruvate dioxygenase (4-HPPD) and p-hydroxy phenyl pyruvate dioxygenase (p-HPPD) are synonymous.

“HPPD herbicides” are herbicides that are bleachers and whose primary site of action is HPPD. Many are well known and described elsewhere herein and in the lature (Hawkes “Hydroxyphenylpyruvate Dioxygenase (HPPD)—The Herbicide Target.” In Modern Crop Protection Compounds. Eds. Krämer and Schirmer. Weinheim, Germany: Wiley-VCH, 2007. Ch. 4.2, pp. 211-220; Edmunds “Hydroxyphenylpyruvate dioxygenase (HPPD) Inhibitors: Triketones.” In Modern Crop Protection Compounds. Eds. Krämer and Schirmer. Weinheim, Germany: Wiley-VCH, 2007. Ch. 4.2, pp. 221-242). As used herein, the term “HPPD herbicides” refers to herbicides that act either directly or indirectly to inhibit HPPD, where the herbicides are bleachers and where inhibition of HPPD is at least part of the herbicide's mode of action on plants.

As used herein, plants which are substantially “tolerant” to a herbicide exhibit, when treated with said herbicide, a dose/response curve which is shifted to the right when compared with that exhibited by similarly subjected non tolerant like plants. Such dose/response curves have “dose” plotted on the x-axis and “percentage kill or damage”, “herbicidal effect” etc. plotted on the y-axis. Tolerant plants will typically require at least twice as much herbicide as non tolerant like plants in order to produce a given herbicidal effect. Plants which are substantially “resistant” to the herbicide exhibit few, if any, necrotic, lytic, chlorotic or other lesions or, at least, none that impact significantly on yield, when subjected to the herbicide at concentrations and rates which are typically employed by the agricultural community to kill weeds in the field.

As used herein, “non-transgenic-like plants” are plants that are similar or the same as transgenic plants but that do not contain a transgene conferring herbicide resistance.

As used herein, the term “confer” refers to providing a characteristic or trait, such as herbicide tolerance or resistance and/or other desirable traits to a plant.

As described elsewhere herein, the term “heterologous” means from another source. In the context of DNA, “heterologous” refers to any foreign “non-self” DNA including that from another plant of the same species. For example, in the present application a soybean HPPD gene that was transgenically expressed back into a soybean plant would still be described as “heterologous” DNA.

The article “a” and “an” are used herein to refer to one or more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one or more element. Throughout the specification the word “comprising,” or variations such as “comprises” or “comprising,” will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

A variety of additional terms are defined or otherwise characterized herein.

HPPD Sequences

The compositions of the invention include isolated or substantially purified native and mutant HPPD polynucleotides and polypeptides as well as host cells comprising the HPPD polynucleotides. Specifically, the present invention provides HPPD polypeptides that have HPPD enzymatic activity and that confer resistance or tolerance in plants to certain classes of herbicides that inhibit HPPD, and variants and fragments thereof. Nucleic acids that encode the native and mutant HPPD polypeptides of the invention are also provided.

Mutant HPPD polypeptides of the presenting invention have amino acid changes at one or more positions relative to the starting wild type sequence from which they are derived, and exhibit enhanced tolerance to one or more HPPD inhibitor herbicides. HPPD enzymes that exhibit enhanced tolerance to an HPPD herbicide may do so by virtue of exhibiting, relative to the like unmutated starting enzyme:

a) a lower K_(m) value for the natural substrate, 4-hydroxyphenylpyruvate;

b) a higher k_(cat) value for converting 4-hydroxyphenylpyruvate to homogentisate;

c) a lower value of the apparent rate constant, k_(on), governing formation of an enzyme: HPPD inhibitor herbicide complex;

d) an increased value of the rate constant, k_(off), governing dissociation of an enzyme: HPPD inhibitor herbicide complex; and/or

e) as a result of changes in one or both of c) and d), an increased value of the equilibrium constant, K_(i) (also called K_(d)), governing dissociation of an enzyme: HPPD inhibitor herbicide complex. DNA sequences encoding such improved mutated HPPDs are used in the provision of HPPD plants, crops, plant cells and seeds of the current invention that offer enhanced tolerance or resistance to one or more HPPD herbicides as compared to like plants likewise expressing the unmutated starting enzyme.

Here it is found that increases in the value of k_(off) are of particular value in improving the ability of an HPPD to confer resistance to a HPPD herbicide whereas, at least in the range above 5000 s⁻¹ M⁻¹ at 25° C. changes in k_(m), have relatively little impact. So, for example, compounds B and C exhibit similar Kd values in respect of the HPPD of SEQ ID NO: 1 but k_(off) values that are about 10 fold different in respect of B and C. Accordingly, transgenic plants expressing the HPPD of SEQ ID NO: 1 exhibit superior resistance to compound B than to compound C.

Thus preferred HPPDs are selected as those that, in comparison with other HPPD enzymes, exhibit superior tolerance to one or more types of HPPD herbicide and where tolerance is characterised in vitro by the numerical value of the parameter (k_(off)×k_(cat)/K_(m HPP)) and where k_(off) is the rate constant governing the dissociation rate of the complex of the HPPD enzyme with herbicide and k_(cat)/K_(m HPP) is the catalytic turnover number divided by the K_(m) value for the substrate HPP (4-hydroxyphenylpyruvate).

Thus in one embodiment of the current invention there is provided an in vitro method for characterising and selecting HPPDs that confer superior levels of tolerance to HPPD herbicides based on measuring and comparing values of k_(cat)/K_(m) HPP and k_(off) or functional equivalents of these parameters.

Site-directed mutations of genes encoding HPPDs are selected so as to encode amino acid changes selected from those listed here either singly or preferably in combination. Genes encoding such mutant forms of HPPDs are useful for making crop plants resistant to herbicides that inhibit HPPD. HPPD genes so modified are especially suitable for use in transgenic plants in order to confer herbicide tolerance or resistance upon crop plants. In a preferred embodiment the HPPDs derive from plants.

Many HPPD sequences are known in the art and can be used to generate mutant HPPD sequences by making amino acid substitutions corresponding to those described herein. For example, a known or suspected HPPD sequence can be inspected for the presence of the amino acid motifs SEQ ID NOs: 15-19 and the corresponding changes described herein made. Alternatively, in the case of HPPDs not deriving from plants the equivalent changes to those indicated here can be made on the basic of sequence line ups and similarity to the motifs specified here. Alternatively, the sequence to be improved by mutation can be aligned with, for example, SEQ ID NO: 1 using standard sequence alignment tools, and the corresponding amino acid substitutions described herein with respect to SEQ ID NO: 1 can be made at the corresponding positions in the reference sequence.

In particular embodiments, the compositions of the invention comprise a mutant HPPD polypeptide having at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to SEQ ID NO: 1 (the HPPD amino acid sequence of Avena sativa) or to SEQ ID NO: 2, or to SEQ ID NO: 3 or to SEQ ID NO: 4 or to SEQ ID NO: 5 or to SEQ ID NO: 6 or to SEQ ID NO: 7 or to SEQ ID NO: 8, where the polypeptide has HPPD enzymatic activity, and where the polypeptide contains one or more substitution(s) corresponding to the amino acid positions listed in column 1 of Table 1.

TABLE 1 Exemplary HPPD mutations Mutable amino acid position relative to SEQ ID NO: 1 Substitution or addition 217 A, I, L, 326 R, K, H, N, I, L, T, S, Q 339 E, D, G, C, N, Q, S, A, L 358 M, F, Y, I, A, W, V 408 R, K, H, E, D, N, Q, A, S, T 411 S, T

In various embodiments, an amino acid at one or more position(s) listed in column 1 is replaced with any other amino acid. In another embodiment, the polypeptide comprises one or more amino acid substitutions, additions, or deletions corresponding to the amino acid substitution(s) or deletion(s) listed in column 2 of Table 1. In yet another embodiment, the polypeptide comprises one or more substitutions corresponding to a conservative variant of the amino acids listed in column 2 of Table 1.

For example, the polypeptide may comprise a mutation corresponding to amino acid position 339 of SEQ ID NO: 1, wherein that amino acid is replaced with a glutamate or a conservative substitution of glutamate.

In particular embodiments, the amino acid sequence of the mutant HPPD polypeptide of the invention is selected from the group consisting of SEQ ID NOs: 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 31, 33, 34, 35, 36, 37, 38, 39, 40, 41, 49 and 50.

The terms “polypeptide,” “peptide,” and “protein” are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residues is an artificial chemical analogue of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers. Polypeptides of the invention can be produced either from a nucleic acid disclosed herein, or by the use of standard molecular biology techniques. For example, a truncated protein of the invention can be produced by expression of a recombinant nucleic acid of the invention in an appropriate host cell, or alternatively by a combination of ex vivo procedures, such as protease digestion and purification.

Accordingly, the present invention also provides nucleic acid molecules comprising polynucleotide sequences that encode mutant HPPD polypeptides that have HPPD enzymatic activity and that confer resistance or tolerance in plants to certain classes of herbicides that inhibit HPPD, and variants and fragments thereof. In general, the invention includes any polynucleotide sequence that encodes any of the mutant HPPD polypeptides described herein, as well as any polynucleotide sequence that encodes HPPD polypeptides having one or more conservative amino acid substitutions relative to the mutant HHPD polypeptides described herein. Conservative substitution tables providing functionally similar amino acids are well known in the art. The following five groups each contain amino acids that are conservative substitutions for one another: Aliphatic: Glycine (G), Alanine (A), Valine (V), Leucine (L), Isoleucine (I); Aromatic: Phenylalanine (F), Tyrosine (Y), Tryptophan (W); Sulfur-containing: Methionine (M), Cysteine (C); Basic: Arginine I, Lysine (K), Histidine (H); Acidic: Aspartic acid (D), Glutamic acid (E), Asparagine (N), Glutamine (Q).

In one embodiment, the present invention provides a polynucleotide sequence encoding an amino acid sequence having at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to SEQ ID NOs: 1, 2, 3, 4, 5, 6, 7 or 8 where the HPPD amino acid sequence derives from a plant, where the polypeptide has HPPD enzymatic activity, and where the polypeptide contains one or more substitutions, additions or deletions as discussed infra.

In another embodiment, the present invention provides a polynucleotide sequence selected from the group consisting of SEQ ID NOs: 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 84 and 85.

As used herein, “nucleic acid” includes reference to a deoxyribonucleotide or ribonucleotide polymer in either single- or double-stranded form, and unless otherwise limited, encompasses known analogues (e.g., peptide nucleic acids) having the essential nature of natural nucleotides in that they hybridize to single-stranded nucleic acids in a manner similar to naturally occurring nucleotides.

As used herein, the terms “encoding” or “encoded” when used in the context of a specified nucleic acid mean that the nucleic acid comprises the requisite information to direct translation of the nucleotide sequence into a specified protein. The information by which a protein is encoded is specified by the use of codons. A nucleic acid encoding a protein may comprise non-translated sequences (e.g., introns) within translated regions of the nucleic acid or may lack such intervening non-translated sequences (e.g., as in cDNA).

The invention encompasses isolated or purified polynucleotide or protein compositions. An “isolated” or “purified” polynucleotide or protein, or biologically active portion thereof, is substantially or essentially free from components that normally accompany or interact with the polynucleotide or protein as found in its naturally occurring environment. Thus, an isolated or purified polynucleotide or protein is substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized. Optimally, an “isolated” polynucleotide is free of sequences (optimally protein encoding sequences) that naturally flank the polynucleotide (i.e., sequences located at the 5′ and 3′ ends of the polynucleotide) in the genomic DNA of the organism from which the polynucleotide is derived. For example, in various embodiments, the isolated polynucleotide can contain less than about 5 kb, 4 kb, 3 kb, 2 kb, 1 kb, 0.5 kb, or 0.1 kb of nucleotide sequence that naturally flank the polynucleotide in genomic DNA of the cell from which the polynucleotide is derived. A protein that is substantially free of interfering enzyme activities and that is capable being characterized in respect of its catalytic, kinetic and molecular properties includes quite crude preparations of protein (for example recombinantly produced in cell extracts) having less than about 98%, 95% 90%, 80%, 70%, 60% or 50% (by dry weight) of contaminating protein as well as preparations further purified by methods known in the art to have 40%, 30%, 20%, 10%, 5%, or 1% (by dry weight) of contaminating protein.

The proteins of the invention may be altered in various ways including amino acid substitutions, deletions, truncations, and insertions. Methods for such manipulations are generally known in the art. For example, amino acid sequence variants and fragments of the mutant HPPD proteins can be prepared by mutations in the DNA. Methods for mutagenesis and polynucleotide alterations are well known in the art. See, for example, Kunkel (1985) Proc. Natl. Acad. Sci. USA 82:488-492; Kunkel et al. (1987) Methods in Enzymol. 154:367-382; U.S. Pat. No. 4,873,192; Walker and Gaastra, eds. (1983) Techniques in Molecular Biology (MacMillan Publishing Company, New York) and the references cited therein. Guidance as to appropriate amino acid substitutions that often do not affect biological activity of the protein of interest may be found in the model of Dayhoff et al. (1978) Atlas of Protein Sequence and Structure (Natl. Biomed. Res. Found., Washington, D.C.). Conservative substitutions, such as exchanging one amino acid with another having similar properties, may be optimal.

The polynucleotides of the invention can also be used to isolate corresponding sequences from other organisms, particularly other plants. In this manner, methods such as PCR, hybridization, and the like can be used to identify such sequences based on their sequence homology to the sequences set forth herein.

In a PCR approach, oligonucleotide primers can be designed for use in PCR reactions to amplify corresponding DNA sequences from cDNA or genomic DNA extracted from any plant of interest. Methods for designing PCR primers and PCR cloning are generally known in the art. See, for example, Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual (2d ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.). See also Innis et al., eds. (1990) PCR Protocols: A Guide to Methods and Applications (Academic Press, New York); Innis and Gelfand, eds. (1995) PCR Strategies (Academic Press, New York); and Innis and Gelfand, eds. (1999) PCR Methods Manual (Academic Press, New York).

In hybridization techniques, all or part of a known polynucleotide is used as a probe that selectively hybridizes to other corresponding polynucleotides present in a population of cloned genomic DNA fragments or cDNA fragments (i.e., genomic or cDNA libraries) from a chosen organism. The hybridization probes may be genomic DNA fragments, cDNA fragments, RNA fragments, or other oligonucleotides, and may be labeled with a detectable group such as ³²P, or any other detectable marker. Methods for preparation of probes for hybridization and for construction of cDNA and genomic libraries are generally known in the art and are disclosed in Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual (2^(nd) ed., Cold Spring Harbor Laboratory Press, Plainview, N.Y.).

By “hybridizing to” or “hybridizing specifically to” refers to the binding, duplexing, or hybridizing of a molecule only to a particular nucleotide sequence under stringent conditions when that sequence is present in a complex mixture (e.g., total cellular) DNA or RNA. “Bind(s) substantially” refers to complementary hybridization between a probe nucleic acid and a target nucleic acid and embraces minor mismatches that can be accommodated by reducing the stringency of the hybridization media to achieve the desired detection of the target nucleic acid sequence.

“Stringent hybridization conditions” and “stringent hybridization wash conditions” in the context of nucleic acid hybridization experiments such as Southern and Northern hybridizations are sequence dependent, and are different under different environmental parameters. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen (1993) Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes part I chapter 2 “Overview of principles of hybridization and the strategy of nucleic acid probe assays” Elsevier, New York. Generally, highly stringent hybridization and wash conditions are selected to be about 5° C. lower than the thermal melting point (T_(m)) for the specific sequence at a defined ionic strength and pH. Typically, under “stringent conditions” a probe will hybridize to its target subsequence, but to no other sequences.

The T_(m) is the temperature (under defined ionic strength and pH) at which 50% of the target sequence hybridizes to a perfectly matched probe. Very stringent conditions are selected to be equal to the T_(m) for a particular probe. An example of stringent hybridization conditions for hybridization of complementary nucleic acids which have more than 100 complementary residues on a filter in a Southern or northern blot is 50% formamide with 1 mg of heparin at 42° C., with the hybridization being carried out overnight. An example of highly stringent wash conditions is 0.1 5M NaCl at 72° C. for about 15 minutes. An example of stringent wash conditions is a 0.2×SSC wash at 65° C. for 15 minutes (see, Sambrook, infra, for a description of SSC buffer). Often, a high stringency wash is preceded by a low stringency wash to remove background probe signal. An example medium stringency wash for a duplex of, e.g., more than 100 nucleotides, is 1×SSC at 45° C. for 15 minutes. An example low stringency wash for a duplex of, e.g., more than 100 nucleotides, is 4-6×SSC at 40° C. for 15 minutes. For short probes (e.g., about 10 to 50 nucleotides), stringent conditions typically involve salt concentrations of less than about 1.0 M Na ion, typically about 0.01 to 1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3, and the temperature is typically at least about 30° C. Stringent conditions can also be achieved with the addition of destabilizing agents such as formamide. In general, a signal to noise ratio of 2× (or higher) than that observed for an unrelated probe in the particular hybridization assay indicates detection of a specific hybridization. Nucleic acids that do not hybridize to each other under stringent conditions are still substantially identical if the proteins that they encode are substantially identical. This occurs, e.g., when a copy of a nucleic acid is created using the maximum codon degeneracy permitted by the genetic code.

The following are examples of sets of hybridization/wash conditions that may be used to clone nucleotide sequences that are homologues of reference nucleotide sequences of the present invention: a reference nucleotide sequence preferably hybridizes to the reference nucleotide sequence in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO₄, 1 mM EDTA at 50° C. with washing in 2×SSC, 0.1% SDS at 50° C., more desirably in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO₄, 1 mM EDTA at 50° C. with washing in 1×SSC, 0.1% SDS at 50° C., more desirably still in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO₄, 1 mM EDTA at 50° C. with washing in 0.5×SSC, 0.1% SDS at 50° C., preferably in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO₄, 1 mM EDTA at 50° C. with washing in 0.1×SSC, 0.1% SDS at 50° C., more preferably in 7% sodium dodecyl sulfate (SDS), 0.5 M NaPO₄, 1 mM EDTA at 50° C. with washing in 0.1×SSC, 0.1% SDS at 65° C.

Fragments and variants of the disclosed nucleotide sequences and proteins encoded thereby are also encompassed by the present invention. “Fragment” is intended to mean a portion of the nucleotide sequence or a portion of the amino acid sequence and hence protein encoded thereby. Fragments of a nucleotide sequence may encode protein fragments that retain the biological activity of the mutant HPPD protein and hence have HPPD enzymatic activity. Alternatively, fragments of a nucleotide sequence that are useful as hybridization probes or in mutagenesis and shuffling reactions to generate yet further HPPD variants generally do not encode fragment proteins retaining biological activity. Thus, fragments of a nucleotide sequence may range from at least about 20 nucleotides, about 50 nucleotides, about 100 nucleotides, and up to the full-length nucleotide sequence encoding the polypeptides of the invention.

A fragment of a nucleotide sequence that encodes a biologically active portion of a mutant HPPD protein of the invention will encode at least 15, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 150, 180, 200, 250, 300, 350 contiguous amino acids, or up to the total number of amino acids present in a full-length mutant HPPD polypeptide of the invention. Fragments of a nucleotide sequence that are useful as hybridization probes or PCR primers generally need not encode a biologically active portion of an HPPD protein.

As used herein, “full-length sequence” in reference to a specified polynucleotide means having the entire nucleic acid sequence of a native or mutated HPPD sequence. “Native sequence” is intended to mean an endogenous sequence, i.e., a non-engineered sequence found in an organism's genome.

Thus, a fragment of a nucleotide sequence of the invention may encode a biologically active portion of a mutant HPPD polypeptide, or it may be a fragment that can be used as a hybridization probe etc. or PCR primer using methods disclosed below. A biologically active portion of a mutant HPPD polypeptide can be prepared by isolating a portion of one of the nucleotide sequences of the invention, expressing the encoded portion of the mutant HPPD protein (e.g., by recombinant expression in vitro), and assessing the activity of the encoded portion of the mutant HPPD protein. Nucleic acid molecules that are fragments of a nucleotide sequence of the invention comprise at least 15, 20, 50, 75, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, or 1300 contiguous nucleotides, or up to the number of nucleotides present in a full-length nucleotide sequence disclosed herein.

“Variants” is intended to mean substantially similar sequences. For polynucleotides, a variant comprises a deletion and/or addition of one or more nucleotides at one or more internal sites within the reference polynucleotide and/or a substitution of one or more nucleotides at one or more sites in the mutant HPPD polynucleotide. As used herein, a “reference” polynucleotide or polypeptide comprises a mutant HPPD nucleotide sequence or amino acid sequence, respectively. As used herein, a “native” polynucleotide or polypeptide comprises a naturally occurring nucleotide sequence or amino acid sequence, respectively. One of skill in the art will recognize that variants of the nucleic acids of the invention will be constructed such that the open reading frame is maintained. For polynucleotides, conservative variants include those sequences that, because of the degeneracy of the genetic code, encode the amino acid sequence of one of the mutant HPPD polypeptides of the invention. Naturally occurring allelic variants such as these can be identified with the use of well-known molecular biology techniques, as, for example, with polymerase chain reaction (PCR) and hybridization techniques as outlined below. Variant polynucleotides also include synthetically derived polynucleotide, such as those generated, for example, by using site-directed mutagenesis but which still encode a mutant HPPD protein of the invention. Generally, variants of a particular polynucleotide of the invention will have at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity to that particular polynucleotide as determined by sequence alignment programs and parameters described elsewhere herein.

Variants of a particular polynucleotide of the invention (i.e., the reference polynucleotide) can also be evaluated by comparison of the percent sequence identity between the polypeptide encoded by a variant polynucleotide and the polypeptide encoded by the reference polynucleotide. Thus, for example, a polynucleotide that encodes a polypeptide with a given percent sequence identity to the polypeptides of SEQ ID NOs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 49 and 50 are disclosed. Percent sequence identity between any two polypeptides can be calculated using sequence alignment programs and parameters described elsewhere herein. Where any given pair of polynucleotides of the invention is evaluated by comparison of the percent sequence identity shared by the two polypeptides they encode, the percent sequence identity between the two encoded polypeptides is at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity across the entirety of the HPPD sequences described herein.

“Variant” protein is intended to mean a protein derived from the reference protein by deletion or addition of one or more amino acids at one or more internal sites in the mutant HPPD protein and/or substitution of one or more amino acids at one or more sites in the mutant HPPD protein. Variant proteins encompassed by the present invention are biologically active, that is they continue to possess the desired biological activity of the mutant HPPD protein, that is, HPPD enzymatic activity and/or herbicide tolerance as described herein. Such variants may result from, for example, genetic polymorphism or from human manipulation. Biologically active variants of a mutant HPPD protein of the invention will have at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity across the entirety of the amino acid sequence for the mutant HPPD protein as determined by sequence alignment programs and parameters described elsewhere herein. A biologically active variant of a protein of the invention may differ from that protein by as few as 1-15 amino acid residues, as few as 1-10, such as 6-10, as few as 5, as few as 4, 3, 2, or even 1 amino acid residue.

Methods of alignment of sequences for comparison are well known in the art and can be accomplished using mathematical algorithms such as the algorithm of Myers and Miller (1988) CABIOS 4:11-17; the local alignment algorithm of Smith et al. (1981) Adv. Appl. Math. 2:482; the global alignment algorithm of Needleman and Wunsch (1970) J Mol. Biol. 48:443-453; and the algorithm of Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 872264, modified as in Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5873-5877. Computer implementations of these mathematical algorithms can be utilized for comparison of sequences to determine sequence identity. Such implementations include, but are not limited to: CLUSTAL in the PC/Gene program (available from Intelligenetics, Mountain View, Calif.); the ALIGN program (Version 2.0) and GAP, BESTFIT, BLAST, FASTA, and TFASTA in the GCG Wisconsin Genetics Software Package, Version 10 (available from Accelrys Inc., 9685 Scranton Road, San Diego, Calif., USA).

Gene Stacking

In certain embodiments the polynucleotides of the invention encoding native or mutant HPPD polypeptides or variants thereof that retain HPPD enzymatic activity (e.g., a polynucleotide sequence encoding an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 2, 3, 4, 5, 6, 7, 8, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 49 and 50) can be stacked with any combination of polynucleotide sequences of interest in order to create plants with a desired trait. A trait, as used herein, refers to the phenotype derived from a particular sequence or groups of sequences. For example, the polynucleotides encoding a mutant HPPD polypeptide or variant thereof that retains HPPD enzymatic activity may be stacked with any other polynucleotides encoding polypeptides that confer a desirable trait, including but not limited to resistance to diseases, insects, and herbicides, tolerance to heat and drought, reduced time to crop maturity, improved industrial processing, such as for the conversion of starch or biomass to fermentable sugars, and improved agronomic quality, such as high oil content and high protein content.

In a particular embodiment of the invention polynucleotides may be stacked (or, alternatively, expression cassettes may be stacked on a single polynucleotide) so as to express more than one type of HPPD polypeptide within a plant. This is a particular advantage where, for example, one HPPD is particularly suitable for providing resistance to one class of HPPD herbicide while the other provides better tolerance to a different class of HPPD herbicide. Stacking HPPD polypeptides is also an advantage where one polypeptide expresses inherent herbicide-resistance but is somewhat labile. This herbicide-resistant HPPD can then be stabilised in mixed expression with, for example, similar but less temperature-labile HPPDs through the formation of mixed enzyme dimers.

Exemplary polynucleotides that may be stacked with polynucleotides of the invention encoding a mutant HPPD polypeptide or variant thereof that retains HPPD enzymatic activity include polynucleotides encoding polypeptides conferring resistance to pests/pathogens such as viruses, nematodes, insects or fungi, and the like. Exemplary polynucleotides that may be stacked with polynucleotides of the invention include polynucleotides encoding: polypeptides having pesticidal and/or insecticidal activity, such as other Bacillus thuringiensis toxic proteins (described in U.S. Pat. Nos. 5,366,892; 5,747,450; 5,737,514; 5,723,756; 5,593,881; and Geiser et al. (1986) Gene 48:109), lectins (Van Damme et al. (1994) Plant Mol. Biol. 24:825, pentin (described in U.S. Pat. No. 5,981,722), and the like; traits desirable for disease or herbicide resistance (e.g., fumonisin detoxification genes (U.S. Pat. No. 5,792,931); avirulence and disease resistance genes (Jones et al. (1994) Science 266:789; Martin et al. (1993) Science 262:1432; Mindrinos et al. (1994) Cell 78:1089); a gene encoding an aryloxyalkanoate dioxygenase conferring resistance to certain classes of auxin and acetylCoA carboxylase herbicides (e.g. in PCT Publication Nos. WO 2008/141154, WO 2007/053482 or a tfdA gene giving resistance to 2,4 Din U.S. Pat. No. 6,153,401); a gene encoding a dicamba monoxygenase (Behrens et al. (2007) Science, 316, 1185) conferring resistance to dicamba; a gene encoding a homogentisate solanesyltransferase (HST) conferring resistance to HST-inhibiting herbicides (PCT Publication No. WO 2010/029311); a gene encoding a nitrilase conferring resistance to a nitrile-containing herbicide (e.g. the bxnA bromoxynil nitrilase); acetolactate synthase (ALS) mutants that lead to herbicide resistance such as the S4 and/or Hra mutations; glyphosate resistance (e.g., 5-enol-pyrovyl-shikimate-3-phosphate-synthase (EPSPS) gene, described in U.S. Pat. Nos. 4,940,935 and 5,188,642; or the glyphosate N-acetyltransferase (GAT) gene, described in Castle et al. (2004) Science, 304:1151-1154; and in U.S. Patent Application Publication Nos. 20070004912, 20050246798, and 20050060767)); glufosinate resistance (e.g., phosphinothricin acetyl transferase genes PAT and BAR, described in U.S. Pat. Nos. 5,561,236 and 5,276,268); a cytochrome P450 or variant thereof that confers herbicide resistance or tolerance to, inter alia, HPPD herbicides (U.S. Patent Application Publication No. 20090011936; U.S. Pat. Nos. 6,380,465; 6,121,512; 5,349,127; 6,649,814; and 6,300,544; and PCT Publication No. WO 2007/000077); and traits desirable for processing or process products such as high oil (e.g., U.S. Pat. No. 6,232,529); modified oils (e.g., fatty acid desaturase genes (U.S. Pat. No. 5,952,544; PCT Publication No. WO 94/11516)); modified starches (e.g., ADPG pyrophosphorylases (AGPase), starch synthases (SS), starch branching enzymes (SBE), and starch debranching enzymes (SDBE)); and polymers or bioplastics (e.g., U.S. Pat. No. 5,602,321; beta-ketothiolase, polyhydroxybutyrate synthase, and acetoacetyl-CoA reductase (Schubert et al. (1988) J. Bacteriol. 170:5837-5847) facilitate expression of polyhydroxyalkanoates (PHAs)).

Thus, in one embodiment, the polynucleotides encoding a native or mutant HPPD polypeptide or variant thereof that retains HPPD enzymatic activity are stacked with one or more polynucleotides encoding polypeptides that confer resistance or tolerance to an herbicide. In one embodiment, the desirable trait is resistance or tolerance to an HPPD inhibitor. In another embodiment, the desirable trait is resistance or tolerance to glyphosate. In another embodiment, the desirable trait is resistance or tolerance to glufosinate. In further embodiments the desirable trait is resistance or tolerance to an HST inhibitor herbicide, an auxin herbicide or a PSII herbicide.

These stacked combinations can be created by any method including, but not limited to, cross-breeding plants by any conventional or TopCross methodology, or genetic transformation. If the sequences are stacked by genetically transforming the plants, the polynucleotide sequences of interest can be combined at any time and in any order. For example, a transgenic plant comprising one or more desired traits can be used as the target to introduce further traits by subsequent transformation. The traits can be introduced simultaneously in a co-transformation protocol with the polynucleotides of interest provided by any combination of transformation cassettes. For example, if two sequences will be introduced, the two sequences can be contained in separate transformation cassettes (trans) or contained on the same transformation cassette (cis). Expression of the sequences can be driven by the same promoter or by different promoters. In certain cases, it may be desirable to introduce a transformation cassette that will suppress the expression of the polynucleotide of interest. This may be combined with any combination of other suppression cassettes or overexpression cassettes to generate the desired combination of traits in the plant. It is further recognized that polynucleotide sequences can be stacked at a desired genomic location using a site-specific recombination system. See, for example, PCT Publication Nos. WO 99/25821, WO 99/25854, WO 99/25840, WO 99/25855, and WO 99/25853.

Plant Expression Cassettes

The compositions of the invention may additionally contain nucleic acid sequences for transformation and expression in a plant of interest. The nucleic acid sequences may be present in DNA constructs or expression cassettes. “Expression cassette” as used herein means a nucleic acid molecule capable of directing expression of a particular nucleotide sequence in an appropriate host cell, comprising a promoter operatively linked to the nucleotide sequence of interest (i.e., a polynucleotide encoding a mutant HPPD polypeptide or variant thereof that retains HPPD enzymatic activity, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits) which is operatively linked to termination signals. It also typically comprises sequences required for proper translation of the nucleotide sequence. The coding region usually codes for a protein of interest but may also code for a functional RNA of interest, for example antisense RNA or a nontranslated RNA, in the sense or antisense direction. The expression cassette comprising the nucleotide sequence of interest may be chimeric, meaning that at least one of its components is heterologous with respect to at least one of its other components. The expression cassette may also be one that is naturally occurring but has been obtained in a recombinant form useful for heterologous expression. Typically, however, the expression cassette is heterologous with respect to the host, i.e., the particular DNA sequence of the expression cassette does not occur naturally in the host cell and must have been introduced into the host cell or an ancestor of the host cell by a transformation event. The expression of the nucleotide sequence in the expression cassette may be under the control of a constitutive promoter or of an inducible promoter that initiates transcription only when the host cell is exposed to some particular external stimulus. Additionally, the promoter can also be specific to a particular tissue or organ or stage of development.

The present invention encompasses the transformation of plants with expression cassettes capable of expressing a polynucleotide of interest, i.e., a polynucleotide encoding a mutant HPPD polypeptide or variant thereof that retains HPPD enzymatic activity, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits. The expression cassette will include in the 5′-3′ direction of transcription, a transcriptional and translational initiation region (i.e., a promoter) and a polynucleotide open reading frame. The expression cassette may optionally comprise a transcriptional and translational termination region (i.e., termination region) functional in plants. In some embodiments, the expression cassette comprises a selectable marker gene to allow for selection for stable transformants. Expression constructs of the invention may also comprise a leader sequence and/or a sequence allowing for inducible expression of the polynucleotide of interest. See, Guo et al. (2003) Plant 34:383-92 and Chen et al. (2003) Plant 36:731-40 for examples of sequences allowing for inducible expression.

The regulatory sequences of the expression construct are operably linked to the polynucleotide of interest. By “operably linked” is intended a functional linkage between a promoter and a second sequence wherein the promoter sequence initiates and mediates transcription of the DNA sequence corresponding to the second sequence. Generally, operably linked means that the nucleotide sequences being linked are contiguous.

Any promoter capable of driving expression in the plant of interest may be used in the practice of the invention. The promoter may be native or analogous or foreign or heterologous to the plant host. The terms “heterologous” and “exogenous” when used herein to refer to a nucleic acid sequence (e.g. a DNA or RNA sequence) or a gene, refer to a sequence that originates from a source foreign to the particular host cell or, if from the same source, is modified from its original form. Thus, a heterologous gene in a host cell includes a gene that is endogenous to the particular host cell but has been modified through, for example, the use of DNA shuffling. The terms also include non-naturally occurring multiple copies of a naturally occurring DNA sequence. Thus, the terms refer to a DNA segment that is foreign or heterologous to the cell, or homologous to the cell but in a position within the host cell nucleic acid in which the element is not ordinarily found. Exogenous DNA segments are expressed to yield exogenous polypeptides.

A “homologous” nucleic acid (e.g., DNA) sequence is a nucleic acid (e.g., DNA or RNA) sequence naturally associated with a host cell into which it is introduced.

The choice of promoters to be included depends upon several factors, including, but not limited to, efficiency, selectability, inducibility, desired expression level, and cell- or tissue-preferential expression. It is a routine matter for one of skill in the art to modulate the expression of a sequence by appropriately selecting and positioning promoters and other regulatory regions relative to that sequence. The promoters that are used for expression of the transgene(s) can be a strong plant promoter, a viral promoter, or a chimeric promoters composed of elements such as: TATA box from any gene (or synthetic, based on analysis of plant gene TATA boxes), optionally fused to the region 5′ to the TATA box of plant promoters (which direct tissue and temporally appropriate gene expression), optionally fused to 1 or more enhancers (such as the 35S enhancer, FMV enhancer, CMP enhancer, RUBISCO SMALL SUBUNIT enhancer, PLASTOCYANIN enhancer).

Exemplary constitutive promoters include, for example, the core promoter of the Rsyn7 promoter and other constitutive promoters disclosed in WO 99/43838 and U.S. Pat. No. 6,072,050; the core CaMV 35S promoter (Odell et al. (1985) Nature 313:810-812); rice actin (McElroy et al. (1990) Plant Cell 2:163-171); ubiquitin (Christensen et al. (1989) Plant Mol. Biol. 12:619-632 and Christensen et al. (1992) Plant Mol. Biol. 18:675-689); pEMU (Last et al. (1991) Theor. Appl. Genet. 81:581-588); MAS (Velten et al. (1984) EMBO J. 3:2723-2730); ALS promoter (U.S. Pat. No. 5,659,026), and the like. Other constitutive promoters are included in, for example, U.S. Pat. Nos. 5,608,149; 5,608,144; 5,604,121; 5,569,597; 5,466,785; 5,399,680; 5,268,463; 5,608,142; and 6,177,611.

Appropriate plant or chimeric promoters are useful for applications such as expression of transgenes in certain tissues, while minimizing expression in other tissues, such as seeds, or reproductive tissues. Exemplary cell type- or tissue-preferential promoters drive expression preferentially in the target tissue, but may also lead to some expression in other cell types or tissues as well. Methods for identifying and characterizing promoter regions in plant genomic DNA include, for example, those described in the following references: Jordano, et al., Plant Cell, 1:855-866 (1989); Bustos, et al., Plant Cell, 1:839-854 (1989); Green, et al., EMBO J. 7, 4035-4044 (1988); Meier, et al., Plant Cell, 3, 309-316 (1991); and Zhang, et al., Plant Physiology 110: 1069-1079 (1996).

In other embodiments of the present invention, inducible promoters may be desired. Inducible promoters drive transcription in response to external stimuli such as chemical agents or environmental stimuli. For example, inducible promoters can confer transcription in response to hormones such as giberellic acid or ethylene, or in response to light or drought.

A variety of transcriptional terminators are available for use in expression cassettes. These are responsible for the termination of transcription beyond the transgene and correct mRNA polyadenylation. The termination region may be native with the transcriptional initiation region, may be native with the operably linked DNA sequence of interest, may be native with the plant host, or may be derived from another source (i.e., foreign or heterologous to the promoter, the DNA sequence of interest, the plant host, or any combination thereof). Appropriate transcriptional terminators are those that are known to function in plants and include the CAMV 35S terminator, the tml terminator, the nopaline synthase terminator and the pea rbcs E9 terminator. These can be used in both monocotyledons and dicotyledons. In addition, a gene's native transcription terminator may be used.

Generally, the expression cassette will comprise a selectable marker gene for the selection of transformed cells. Selectable marker genes are utilized for the selection of transformed cells or tissues.

Numerous sequences have been found to enhance gene expression from within the transcriptional unit and these sequences can be used in conjunction with the genes of this invention to increase their expression in transgenic plants.

Various intron sequences have been shown to enhance expression, particularly in monocotyledonous cells. For example, the introns of the maize Adhl gene have been found to significantly enhance the expression of the wild-type gene under its cognate promoter when introduced into maize cells. Intron 1 was found to be particularly effective and enhanced expression in fusion constructs with the chloramphenicol acetyltransferase gene (Callis et al., Genes Develop. 1:1183-1200 (1987)). In the same experimental system, the intron from the maize bronze 1 gene had a similar effect in enhancing expression. Intron sequences have been routinely incorporated into plant transformation vectors, typically within the non-translated leader.

A number of non-translated leader sequences derived from viruses are also known to enhance expression, and these are particularly effective in dicotyledonous cells. Specifically, leader sequences from tobacco mosaic virus (TMV, the “W-sequence”), maize chlorotic mottle virus (MCMV), and alfalfa mosaic virus (AMV) have been shown to be effective in enhancing expression (e.g., Gallie et al. Nucl. Acids Res. 15: 8693-8711 (1987); Skuzeski et al. Plant Molec. Biol. 15: 65-79 (1990)). Other leader sequences known in the art include but are not limited to: picomavirus leaders, for example, EMCV leader (encephalomyocarditis 5′ noncoding region) (Elroy-Stein, O., Fuerst, T. R., and Moss, B. PNAS USA 86:6126-6130 (1989)); potyvirus leaders, for example, tobacco etch virus (TEV) leader (Allison et al., 1986); maize dwarf mosaic virus (MDMV) leader; Virology 154:9-20); human immunoglobulin heavy-chain binding protein (BiP) leader, (Macejak, D. G., and Samow, P., Nature 353: 90-94 (1991); untranslated leader from the coat protein mRNA of alfalfa mosaic virus (AMV RNA 4), (Jobling, S. A., and Gehrke, L., Nature 325:622-625 (1987); tobacco mosaic virus leader (TMV), (Gallie, D. R. et al., Molecular Biology of RNA, 237-256 (1989); and maize chlorotic mottle virus leader (MCMV) (Lommel, S. A. et al., Virology 81:382-385 (1991). See also, Della-Cioppa et al., Plant Physiology 84:965-968 (1987).

The present invention also relates to nucleic acid constructs comprising one or more of the expression cassettes described above. The construct can be a vector, such as a plant transformation vector. In one embodiment, the vector is a plant transformation vector comprising a polynucleotide comprising the sequence set forth in SEQ ID NO: 51 or SEQ ID NO: 52.

Plants

As used herein, the term “plant part” or “plant tissue” includes plant cells, plant protoplasts, plant cell tissue cultures from which plants can be regenerated, plant calli, plant clumps, and plant cells that are intact in plants or parts of plants such as embryos, pollen, ovules, seeds, leaves, flowers, branches, fruit, kernels, ears, cobs, husks, stalks, roots, root tips, anthers, and the like. The aforementioned term also includes plant products, such as grain, fruits, and nuts.

Plants useful in the present invention include plants that are transgenic for at least a polynucleotide encoding a mutant HPPD polypeptide or variant thereof that retains HPPD enzymatic activity, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits. The type of plant selected depends on a variety of factors, including for example, the downstream use of the harvested plant material, amenability of the plant species to transformation, and the conditions under which the plants will be grown, harvested, and/or processed. One of skill will further recognize that additional factors for selecting appropriate plant varieties for use in the present invention include high yield potential, good stalk strength, resistance to specific diseases, drought tolerance, rapid dry down and grain quality sufficient to allow storage and shipment to market with minimum loss.

Plants according to the present invention include any plant that is cultivated for the purpose of producing plant material that is sought after by man or animal for either oral consumption, or for utilization in an industrial, pharmaceutical, or commercial process. The invention may be applied to any of a variety of plants, including, but not limited to maize, wheat, rice, barley, soybean, cotton, sorghum, beans in general, rape/canola, alfalfa, flax, sunflower, safflower, millet, rye, sugarcane, sugar beet, cocoa, tea, Brassica, cotton, coffee, sweet potato, flax, peanut, clover; vegetables such as lettuce, tomato, cucurbits, cassava, potato, carrot, radish, pea, lentils, cabbage, cauliflower, broccoli, Brussels sprouts, peppers, and pineapple; tree fruits such as citrus, apples, pears, peaches, apricots, walnuts, avocado, banana, and coconut; and flowers such as orchids, carnations and roses. Other plants useful in the practice of the invention include perennial grasses, such as switchgrass, prairie grasses, indiangrass, big bluestem grass and the like. It is recognized that mixtures of plants may be used.

In addition, the term “crops” is to be understood as also including crops that have been rendered tolerant to herbicides or classes of herbicides (such as, for example, ALS inhibitors, for example primisulfuron, prosulfuron and trifloxysulfuron, EPSPS (5-enol-pyrovyl-shikimate-3-phosphate-synthase) inhibitors, GS (glutamine synthetase) inhibitors) as a result of conventional methods of breeding or genetic engineering. Examples of crops that have been rendered tolerant to herbicides or classes of herbicides by genetic engineering methods include glyphosate- and glufosinate-resistant crop varieties commercially available under the trade names RoundupReady® and LibertyLink®. The method according to the present invention is especially suitable for the protection of soybean crops which have also been rendered tolerant to glyphosate and/or glufosinate and where HPPD herbicides are used in a weed control programme along with other such herbicides (glufosinate and/or glyphosate) for weed control.

It is further contemplated that the constructs of the invention may be introduced into plant varieties having improved properties suitable or optimal for a particular downstream use. For example, naturally-occurring genetic variability results in plants with resistance or tolerance to HPPD inhibitors or other herbicides, and such plants are also useful in the methods of the invention. The method according to the present invention can be further optimized by crossing the transgenes that provide a level of tolerance, with soybean cultivars that exhibit an enhanced level of tolerance to HPPD inhibitors that is found in a small percentage of soybean lines.

Plant Transformation

Once an herbicide resistant or tolerant mutant HPPD polynucleotide, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits, has been cloned into an expression system, it is transformed into a plant cell. The expression cassettes of the present invention can be introduced into the plant cell in a number of art-recognized ways. The term “introducing” in the context of a polynucleotide, for example, a nucleotide construct of interest, is intended to mean presenting to the plant the polynucleotide in such a manner that the polynucleotide gains access to the interior of a cell of the plant. Where more than one polynucleotide is to be introduced, these polynucleotides can be assembled as part of a single nucleotide construct, or as separate nucleotide constructs, and can be located on the same or different transformation vectors. Accordingly, these polynucleotides can be introduced into the host cell of interest in a single transformation event, in separate transformation events, or, for example, in plants, as part of a breeding protocol. The methods of the invention do not depend on a particular method for introducing one or more polynucleotides into a plant, only that the polynucleotide(s) gains access to the interior of at least one cell of the plant. Methods for introducing polynucleotides into plants are known in the art including, but not limited to, transient transformation methods, stable transformation methods, and virus-mediated methods.

“Transient transformation” in the context of a polynucleotide is intended to mean that a polynucleotide is introduced into the plant and does not integrate into the genome of the plant.

By “stably introducing” or “stably introduced” in the context of a polynucleotide introduced into a plant is intended the introduced polynucleotide is stably incorporated into the plant genome, and thus the plant is stably transformed with the polynucleotide.

“Stable transformation” or “stably transformed” is intended to mean that a polynucleotide, for example, a nucleotide construct described herein, introduced into a plant integrates into the genome of the plant and is capable of being inherited by the progeny thereof, more particularly, by the progeny of multiple successive generations.

Numerous transformation vectors available for plant transformation are known to those of ordinary skill in the plant transformation arts, and the genes pertinent to this invention can be used in conjunction with any such vectors. The selection of vector will depend upon the preferred transformation technique and the target species for transformation. For certain target species, different antibiotic or herbicide selection markers may be preferred. Selection markers used routinely in transformation include the nptll gene, which confers resistance to kanamycin and related antibiotics (Messing & Vierra Gene 19: 259-268 (1982); Bevan et al., Nature 304:184-187 (1983)), the pat and bar genes, which confer resistance to the herbicide glufosinate (also called phosphinothricin; see White et al., Nucl. Acids Res 18: 1062 (1990), Spencer et al. Theor. Appl. Genet 79: 625-631 (1990) and U.S. Pat. Nos. 5,561,236 and 5,276,268), the hph gene, which confers resistance to the antibiotic hygromycin (Blochinger & Diggelmann, Mol. Cell Biol. 4: 2929-2931), and the dhfr gene, which confers resistance to methatrexate (Bourouis et al., EMBO J. 2(7): 1099-1104 (1983)), the EPSPS gene, which confers resistance to glyphosate (U.S. Pat. Nos. 4,940,935 and 5,188,642), the glyphosate N-acetyltransferase (GAT) gene, which also confers resistance to glyphosate (Castle et al. (2004) Science, 304:1151-1154; U.S. Patent App. Pub. Nos. 20070004912, 20050246798, and 20050060767); and the mannose-6-phosphate isomerase gene, which provides the ability to metabolize mannose (U.S. Pat. Nos. 5,767,378 and 5,994,629). Alternatively, and in one preferred embodiment the HPPD gene of the current invention is, in combination with the use of an HPPD herbicide as selection agent, itself used as the selectable marker.

Methods for regeneration of plants are also well known in the art. For example, Ti plasmid vectors have been utilized for the delivery of foreign DNA, as well as direct DNA uptake, liposomes, electroporation, microinjection, and microprojectiles. In addition, bacteria from the genus Agrobacterium can be utilized to transform plant cells. Below are descriptions of representative techniques for transforming both dicotyledonous and monocotyledonous plants, as well as a representative plastid transformation technique.

Many vectors are available for transformation using Agrobacterium tumefaciens. These typically carry at least one T-DNA border sequence and include vectors such as pBIN19 (Bevan, Nucl. Acids Res. (1984)). For the construction of vectors useful in Agrobacterium transformation, see, for example, U.S. Patent Application Publication No. 2006/0260011.

Transformation without the use of Agrobacterium tumefaciens circumvents the requirement for T-DNA sequences in the chosen transformation vector and consequently vectors lacking these sequences can be utilized in addition to vectors such as the ones described above which contain T-DNA sequences. Transformation techniques that do not rely on Agrobacterium include transformation via particle bombardment, protoplast uptake (e.g., PEG and electroporation) and microinjection. The choice of vector depends largely on the preferred selection for the species being transformed. For the construction of such vectors, see, e.g., U.S. Patent Application Publication No. 20060260011.

For expression of a nucleotide sequence of the present invention in plant plastids, plastid transformation vector pPH143 (see PCT Publication No. WO 97/32011, Example 36) is used. The nucleotide sequence is inserted into pPH143 thereby replacing the PROTOX coding sequence. This vector is then used for plastid transformation and selection of transformants for spectinomycin resistance. Alternatively, the nucleotide sequence is inserted in pPH143 so that it replaces the aadH gene. In this case, transformants are selected for resistance to PROTOX inhibitors.

Transformation techniques for dicotyledons are well known in the art and include Agrobacterium-based techniques and techniques that do not require Agrobacterium. Non-Agrobacterium techniques involve the uptake of exogenous genetic material directly by protoplasts or cells. This can be accomplished by PEG or electroporation mediated uptake, particle bombardment-mediated delivery, or microinjection. Examples of these techniques are described by Paszkowski et al., EMBO 1 3: 2717-2722 (1984), Potrykus et al., Mol. Gen. Genet. 199: 169-177 (1985), Reich et al., Biotechnology 4: 1001-1004 (1986), and Klein et al., Nature 327: 70-73 (1987). In each case the transformed cells are regenerated to whole plants using standard techniques known in the art.

Agrobacterium-mediated transformation is a preferred technique for transformation of dicotyledons because of its high efficiency of transformation and its broad utility with many different species. Agrobacterium transformation typically involves the transfer of the binary vector carrying the foreign DNA of interest (e.g., pCIB200 or pCIB2001) to an appropriate Agrobacterium strain which may depend of the complement of vir genes carried by the host Agrobacterium strain either on a co-resident Ti plasmid or chromosomally (e.g., strain CIB542 for pCIB200 and pCIB2001 (Uknes et al. Plant Cell 5: 159-169 (1993)). The transfer of the recombinant binary vector to Agrobacterium is accomplished by a triparental mating procedure using E. coli carrying the recombinant binary vector, a helper E. coli strain which carries a plasmid such as pRK2013 and which is able to mobilize the recombinant binary vector to the target Agrobacterium strain. Alternatively, the recombinant binary vector can be transferred to Agrobacterium by DNA transformation (Hofgen & Willmitzer, Nucl. Acids Res. 16: 9877 (1988)).

Transformation of the target plant species by recombinant Agrobacterium usually involves co-cultivation of the Agrobacterium with explants from the plant and follows protocols well known in the art. Transformed tissue is regenerated on selectable medium carrying the antibiotic or herbicide resistance marker present between the binary plasmid T-DNA borders.

Another approach to transforming plant cells with a gene involves propelling inert or biologically active particles at plant tissues and cells. This technique is disclosed in U.S. Pat. Nos. 4,945,050, 5,036,006, and 5,100,792 all to Sanford et al. Generally, this procedure involves propelling inert or biologically active particles at the cells under conditions effective to penetrate the outer surface of the cell and afford incorporation within the interior thereof. When inert particles are utilized, the vector can be introduced into the cell by coating the particles with the vector containing the desired gene. Alternatively, the target cell can be surrounded by the vector so that the vector is carried into the cell by the wake of the particle. Biologically active particles (e.g., dried yeast cells, dried bacterium or a bacteriophage, each containing DNA sought to be introduced) can also be propelled into plant cell tissue.

Transformation of most monocotyledon species has now also become routine. Preferred techniques include direct gene transfer into protoplasts using PEG or electroporation techniques, and particle bombardment into callus tissue. Transformations can be undertaken with a single DNA species or multiple DNA species (i.e., co-transformation) and both of these techniques are suitable for use with this invention. Co-transformation may have the advantage of avoiding complete vector construction and of generating transgenic plants with unlinked loci for the gene of interest and the selectable marker, enabling the removal of the selectable marker in subsequent generations, should this be regarded desirable. However, a disadvantage of the use of co-transformation is the less than 100% frequency with which separate DNA species are integrated into the genome (Schocher et al. Biotechnology 4: 1093-1096 (1986)).

European patents EP 0 292 435 and EP 0 392 225, and PCT Publication No. WO 93/07278 describe techniques for the preparation of callus and protoplasts from an elite inbred line of maize, transformation of protoplasts using PEG or electroporation, and the regeneration of maize plants from transformed protoplasts. Gordon-Kamm et al. (Plant Cell 2: 603-618 (1990)) and Fromm et al. (Biotechnology 8: 833-839 (1990)) have published techniques for transformation of A188-derived maize line using particle bombardment. Furthermore, PCT Publication No. WO 93/07278 and Koziel et al. (Biotechnology 11: 194-200 (1993)) describe techniques for the transformation of elite inbred lines of maize by particle bombardment. This technique utilizes immature maize embryos of 1.5-2.5 mm length excised from a maize ear 14-15 days after pollination and a PDS-1000He Biolistics device for bombardment.

Transformation of rice can also be undertaken by direct gene transfer techniques utilizing protoplasts or particle bombardment. Protoplast-mediated transformation has been described for Japonica-types and Indica-types (Zhang et al. Plant Cell Rep 7: 379-384 (1988); Shimamoto et al. Nature 338: 274-277 (1989); Datta et al. Biotechnology 8:736-740 (1990)). Both types are also routinely transformable using particle bombardment (Christou et al. Biotechnology 9: 957-962 (1991)). Furthermore, PCT Publication No. WO 93/21335 describes techniques for the transformation of rice via electroporation.

European patent EP 0 332 581 describes techniques for the generation, transformation and regeneration of Pooideae protoplasts. These techniques allow the transformation of Dactyl's and wheat. Furthermore, wheat transformation has been described by Vasil et al. (Biotechnology 10: 667-674 (1992)) using particle bombardment into cells of type C long-term regenerable callus, and also by Vasil et al. (Biotechnology 11:1553-1558 (1993)) and Weeks et al. (Plant Physiol. 102:1077-1084 (1993)) using particle bombardment of immature embryos and immature embryo-derived callus. A preferred technique for wheat transformation, however, involves the transformation of wheat by particle bombardment of immature embryos and includes either a high sucrose or a high maltose step prior to gene delivery. Prior to bombardment, any number of embryos (0.75-1 mm in length) are plated onto MS medium with 3% sucrose (Murashiga & Skoog, Physiologia Plantarum 15: 473-497 (1962)) and 3 mg/l 2,4-D for induction of somatic embryos, which is allowed to proceed in the dark. On the chosen day of bombardment, embryos are removed from the induction medium and placed onto the osmoticum (i.e. induction medium with sucrose or maltose added at the desired concentration, typically 15%). The embryos are allowed to plasmolyze for 2-3 hours and are then bombarded. Twenty embryos per target plate is typical, although not critical. An appropriate gene-carrying plasmid (such as pCIB3064 or pSOG35) is precipitated onto micrometer size gold particles using standard procedures. Each plate of embryos is shot with the DuPont BIOLISTICS® helium device using a burst pressure of about 1000 psi using a standard 80 mesh screen. After bombardment, the embryos are placed back into the dark to recover for about 24 hours (still on osmoticum). After 24 hrs, the embryos are removed from the osmoticum and placed back onto induction medium where they stay for about a month before regeneration. Approximately one month later the embryo explants with developing embryogenic callus are transferred to regeneration medium (MS+1 mg/l NAA, 5 mg/l GA), further containing the appropriate selection agent (10 mg/l basta in the case of pCIB3064 and 2 mg/l methotrexate in the case of pSOG35). After approximately one month, developed shoots are transferred to larger sterile containers known as “GA7s” which contain half-strength MS, 2% sucrose, and the same concentration of selection agent.

Transformation of monocotyledons using Agrobacterium has also been described. See, PCT Publication No. WO 94/00977, U.S. Pat. No. 5,591,616, and Negrotto et al., Plant Cell Reports 19: 798-803 (2000). For example, rice (Oryza sativa) can be used for generating transgenic plants. Various rice cultivars can be used (Hiei et al., 1994, Plant Journal 6:271-282; Dong et al., 1996, Molecular Breeding 2:267-276; Hiei et al., 1997, Plant Molecular Biology, 35:205-218). Also, the various media constituents described below may be either varied in quantity or substituted. Embryogenic responses are initiated and/or cultures are established from mature embryos by culturing on MS-CIM medium (MS basal salts, 4.3 g/l; B5 vitamins (200×), 5 ml/l; sucrose, 30 g/l; proline, 500 mg/l; glutamine, 500 mg/l; casein hydrolysate, 300 mg/1; 2,4-D (1 mg/ml), 2 ml/l; adjust pH to 5.8 with 1 N KOH; phytagel, 3 g/l). Either mature embryos at the initial stages of culture response or established culture lines are inoculated and co-cultivated with the Agrobacterium tumefaciens strain LBA4404 (Agrobacterium) containing the desired vector construction. Agrobacterium is cultured from glycerol stocks on solid YPC medium (100 mg/l spectinomycin and any other appropriate antibiotic) for about 2 days at 28° C. Agrobacterium is re-suspended in liquid MS-CIM medium. The Agrobacterium culture is diluted to an OD⁶⁰⁰ of 0.2-0.3 and acetosyringone is added to a final concentration of 200 μM. Acetosyringone is added before mixing the solution with the rice cultures to induce Agrobacterium for DNA transfer to the plant cells. For inoculation, the plant cultures are immersed in the bacterial suspension. The liquid bacterial suspension is removed and the inoculated cultures are placed on co-cultivation medium and incubated at 22° C. for two days. The cultures are then transferred to MS-CIM medium with ticarcillin (400 mg/l) to inhibit the growth of Agrobacterium. For constructs utilizing the PMI selectable marker gene (Reed et al., In Vitro Cell. Dev. Biol.-Plant 37:127-132), cultures are transferred to selection medium containing mannose as a carbohydrate source (MS with 2% mannose, 300 mg/l ticarcillin) after 7 days, and cultured for 3-4 weeks in the dark. Resistant colonies are then transferred to regeneration induction medium (MS with no 2,4-D, 0.5 mg/l IAA, 1 mg/l zeatin, 200 mg/l timentin 2% mannose and 3% sorbitol) and grown in the dark for 14 days. Proliferating colonies are then transferred to another round of regeneration induction media and moved to the light growth room. Regenerated shoots are transferred to GA7 containers with GA7-1 medium (MS with no hormones and 2% sorbitol) for 2 weeks and then moved to the greenhouse when they are large enough and have adequate roots. Plants are transplanted to soil in the greenhouse (T₀ generation) grown to maturity, and the T₁ seed is harvested.

The plants obtained via transformation with a nucleic acid sequence of interest in the present invention can be any of a wide variety of plant species, including those of monocots and dicots; however, the plants used in the method of the invention are preferably selected from the list of agronomically important target crops set forth elsewhere herein. The expression of a gene of the present invention in combination with other characteristics important for production and quality can be incorporated into plant lines through breeding. Breeding approaches and techniques are known in the art. See, for example, Welsh J. R., Fundamentals of Plant Genetics and Breeding, John Wiley & Sons, NY (1981); Crop Breeding, Wood D. R. (Ed.) American Society of Agronomy Madison, Wis. (1983); Mayo O., The Theory of Plant Breeding, Second Edition, Clarendon Press, Oxford (1987); Singh, D. P., Breeding for Resistance to Diseases and Insect Pests, Springer-Verlag, NY (1986); and Wricke and Weber, Quantitative Genetics and Selection Plant Breeding, Walter de Gruyter and Co., Berlin (1986).

For the transformation of plastids, seeds of Nicotiana tabacum c.v. “Xanthienc” are germinated seven per plate in a 1″ circular array on T agar medium and bombarded 12-14 days after sowing with 1 μm tungsten particles (M10, Biorad, Hercules, Calif.) coated with DNA from plasmids pPH143 and pPH145 essentially as described (Svab, Z. and Maliga, P. (1993) PNAS 90, 913-917). Bombarded seedlings are incubated on T medium for two days after which leaves are excised and placed abaxial side up in bright light (350-500 μmol photons/m²/s) on plates of RMOP medium (Svab, Z., Hajdukiewicz, P. and Maliga, P. (1990) PNAS 87, 8526-8530) containing 500 μg/ml spectinomycin dihydrochloride (Sigma, St. Louis, Mo.). Resistant shoots appearing underneath the bleached leaves three to eight weeks after bombardment are subcloned onto the same selective medium, allowed to form callus, and secondary shoots isolated and subcloned. Complete segregation of transformed plastid genome copies (homoplasmicity) in independent subclones is assessed by standard techniques of Southern blotting (Sambrook et al., (1989) Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor). BamHI/EcoRI-digested total cellular DNA (Mettler, I. J. (1987) Plant Mol Biol Reporter 5, 346349) is separated on 1% Tris-borate (TBE) agarose gels, transferred to nylon membranes (Amersham) and probed with ³²P-labeled random primed DNA sequences corresponding to a 0.7 kb BamHI/HindIII DNA fragment from pC8 containing a portion of the rps 7/12 plastid targeting sequence. Homoplasmic shoots are rooted aseptically on spectinomycin-containing MS/IBA medium (McBride, K. E. et al. (1994) PNAS 91, 7301-7305) and transferred to the greenhouse.

The genetic properties engineered into the transgenic seeds and plants described above are passed on by sexual reproduction or vegetative growth and can thus be maintained and propagated in progeny plants. Generally, maintenance and propagation make use of known agricultural methods developed to fit specific purposes such as tilling, sowing or harvesting.

Use of the advantageous genetic properties of the transgenic plants and seeds according to the invention can further be made in plant breeding. Depending on the desired properties, different breeding measures are taken. The relevant techniques are well known in the art and include but are not limited to hybridization, inbreeding, backcross breeding, multi-line breeding, variety blend, interspecific hybridization, aneuploid techniques, etc. Thus, the transgenic seeds and plants according to the invention can be used for the breeding of improved plant lines that, for example, increase the effectiveness of conventional methods such as herbicide or pesticide treatment or allow one to dispense with said methods due to their modified genetic properties.

Many suitable methods for transformation using suitable selection markers such as kanamycin, binary vectors such as from Agrobacterium and plant regeneration as, for example, from tobacco leaf discs are well known in the art. Optionally, a control population of plants are likewise transformed with a polynucleotide expressing the control HPPD. Alternatively, an untransformed dicot plant such as Arabidopsis or tobacco can be used as a control since this, in any case, expresses its own endogenous HPPD.

Herbicide Resistance

The present invention provides transgenic plants, plant cells, tissues, and seeds that have been transformed with a nucleic acid molecule encoding a mutant HPPD or variant thereof that confers resistance or tolerance to herbicides, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits.

In one embodiment, the transgenic plants of the invention exhibit resistance or tolerance to application of herbicide in an amount of from about 5 to about 2,000 grams per hectare (g/ha), including, for example, about 5 g/ha, about 10 g/ha, about 15 g/ha, about 20 g/ha, about 25 g/ha, about 30 g/ha, about 35 g/ha, about 40 g/ha, about 45 g/ha, about 50 g/ha, about 55 g/ha, about 60 g/ha, about 65 g/ha, about 70 g/ha, about 75 g/ha, about 80 g/ha, about 85 g/ha, about 90 g/ha, about 95 g/ha, about 100 g/ha, about 110 g/ha, about 120 g/ha, about 130 g/ha, about 140 g/ha, about 150 g/ha, about 160 g/ha, about 170 g/ha, about 180 g/ha, about 190 g/ha, about 200 g/ha, about 210 g/ha, about 220 g/ha, about 230 g/ha, about 240 g/ha, about 250 g/ha, about 260 g/ha, about 270 g/ha, about 280 g/ha, about 290 g/ha, about 300 g/ha, about 310 g/ha, about 320 g/ha, about 330 g/ha, about 340 g/ha, about 350 g/ha, about 360 g/ha, about 370 g/ha, about 380 g/ha, about 390 g/ha, about 400 g/ha, about 410 g/ha, about 420 g/ha, about 430 g/ha, about 440 g/ha, about 450 g/ha, about 460 g/ha, about 470 g/ha, about 480 g/ha, about 490 g/ha, about 500 g/ha, about 510 g/ha, about 520 g/ha, about 530 g/ha, about 540 g/ha, about 550 g/ha, about 560 g/ha, about 570 g/ha, about 580 g/ha, about 590 g/ha, about 600 g/ha, about 610 g/ha, about 620 g/ha, about 630 g/ha, about 640 g/ha, about 650 g/ha, about 660 g/ha, about 670 g/ha, about 680 g/ha, about 690 g/ha, about 700 g/ha, about 710 g/ha, about 720 g/ha, about 730 g/ha, about 740 g/ha, about 750 g/ha, about 760 g/ha, about 770 g/ha, about 780 g/ha, about 790 g/ha, about 800 g/ha, about 810 g/ha, about 820 g/ha, about 830 g/ha, about 840 g/ha, about 850 g/ha, about 860 g/ha, about 870 g/ha, about 880 g/ha, about 890 g/ha, about 900 g/ha, about 910 g/ha, about 920 g/ha, about 930 g/ha, about 940 g/ha, about 950 g/ha, about 960 g/ha, about 970 g/ha, about 980 g/ha, about 990 g/ha, about 1,000, g/ha, about 1,010 g/ha, about 1,020 g/ha, about 1,030 g/ha, about 1,040 g/ha, about 1,050 g/ha, about 1,060 g/ha, about 1,070 g/ha, about 1,080 g/ha, about 1,090 g/ha, about 1,100 g/ha, about 1,110 g/ha, about 1,120 g/ha, about 1,130 g/ha, about 1,140 g/ha, about 1,150 g/ha, about 1,160 g/ha, about 1,170 g/ha, about 1,180 g/ha, about 1,190 g/ha, about 1,200 g/ha, about 1,210 g/ha, about 1,220 g/ha, about 1,230 g/ha, about 1,240 g/ha, about 1,250 g/ha, about 1,260 g/ha, about 1,270 g/ha, about 1,280 g/ha, about 1,290 g/ha, about 1,300 g/ha, about 1,310 g/ha, about 1,320 g/ha, about 1,330 g/ha, about 1,340 g/ha, about 1,350 g/ha, about 360 g/ha, about 1,370 g/ha, about 1,380 g/ha, about 1,390 g/ha, about 1,400 g/ha, about 1,410 g/ha, about 1,420 g/ha, about 1,430 g/ha, about 1,440 g/ha, about 1,450 g/ha, about 1,460 g/ha, about 1,470 g/ha, about 1,480 g/ha, about 1,490 g/ha, about 1,500 g/ha, about 1,510 g/ha, about 1,520 g/ha, about 1,530 g/ha, about 1,540 g/ha, about 1,550 g/ha, about 1,560 g/ha, about 1,570 g/ha, about 1,580 g/ha, about 1,590 g/ha, about 1,600 g/ha, about 1,610 g/ha, about 1,620 g/ha, about 1,630 g/ha, about 1,640 g/ha, about 1,650 g/ha, about 1,660 g/ha, about 1,670 g/ha, about 1,680 g/ha, about 1,690 g/ha, about 1,700 g/ha, about 1,710 g/ha, about 1,720 g/ha, about 1,730 g/ha, about 1,740 g/ha, about 1,750 g/ha, about 1,760 g/ha, about 1,770 g/ha, about 1,780 g/ha, about 1,790 g/ha, about 1,800 g/ha, about 1,810 g/ha, about 1,820 g/ha, about 1,830 g/ha, about 1,840 g/ha, about 1,850 g/ha, about 1,860 g/ha, about 1,870 g/ha, about 1,880 g/ha, about 1,890 g/ha, about 1,900 g/ha, about 1,910 g/ha, about 1,920 g/ha, about 1,930 g/ha, about 1,940 g/ha, about 1,950 g/ha, about 1,960 g/ha, about 1,970 g/ha, about 1,980 g/ha, about 1,990 g/ha, or about 2,000.

The average and distribution of herbicide tolerance or resistance levels of a range of primary plant transformation events are evaluated in the normal manner based upon plant damage, meristematic bleaching symptoms etc. at a range of different concentrations of herbicides. These data can be expressed in terms of, for example, GR₅₀ values derived from dose/response curves having “dose” plotted on the x-axis and “percentage kill”, “herbicidal effect”, “numbers of emerging green plants” etc. plotted on the y-axis where increased GR₅₀ values correspond to increased levels of inherent inhibitor-tolerance (e.g., increased k_(off)/K_(mHPP) value) and/or level of expression of the expressed HPPD polypeptide.

The methods of the present invention are especially useful to protect crops from the herbicidal injury of HPPD inhibitor herbicides. For example, the HPPD inhibiting herbicide is suitably selected from the group consisting of bicyclopyrone (CAS RN 352010-68-5), benzobicyclon (CAS RN 156963-66-5), benzofenap (CAS RN 82692-44-2), ketospiradox (CAS RN 192708-91-1) or its free acid (CAS RN 187270-87-7), isoxachlortole (CAS RN 141112-06-3), isoxaflutole (CAS RN 141112-29-0), mesotrione (CAS RN 104206-82-8), pyrasulfotole (CAS RN 365400-11-9), pyrazolynate (CAS RN 58011-68-0), pyrazoxyfen (CAS RN 71561-11-0), sulcotrione (CAS RN 99105-77-8), tefuryltrione (CAS RN 473278-76-1), tembotrione (CAS RN 335104-84-2) and topramezone (CAS RN 210631-68-8); including, where applicable, agrochemically acceptable salts thereof.

Methods of Use

The present invention further provides a method of selectively controlling weeds at a locus comprising crop plants and weeds, wherein the plants are obtained by any of the methods of the current invention described above, wherein the method comprises application to the locus of a weed controlling amount of one or more herbicides. Any of the transgenic plants described herein may be used within these methods of the invention. The term “locus” may include soil, seeds, and seedlings, as well as established vegetation. Herbicides can suitably be applied pre-emergence or post-emergence of the crop or weeds.

The term “weed controlling amount” is meant to include functionally, an amount of herbicide which is capable of affecting the growth or development of a given weed. Thus, the amount may be small enough to simply retard or suppress the growth or development of a given weed, or the amount may be large enough to irreversibly destroy a given weed.

Thus, the present invention provides a method of controlling weeds at a locus comprising applying to the locus a weed-controlling amount of one or more herbicides, where the locus comprises a transgenic plant that has been transformed with a nucleic acid molecule encoding a mutant HPPD polypeptide or variant thereof that confers resistance or tolerance to HPPD herbicides, alone or in combination with one or more additional nucleic acid molecules encoding polypeptides that confer desirable traits. In one embodiment, the desirable trait is resistance or tolerance to an herbicide, including, for example, herbicides selected from the group consisting of an HPPD inhibitor, glyphosate, and glufosinate. In another embodiment, the locus comprises a transgenic plant that has been transformed with any combination of nucleic acid molecules described above, including one or more nucleic acid molecules encoding a mutant HPPD polypeptide or variant thereof that confers resistance or tolerance to an herbicide in combination with at least one, at least two, at least three, or at least four additional nucleic acid molecules encoding polypeptides that confer desirable traits.

In one embodiment, the present invention provides transgenic plants and methods useful for the control of unwanted plant species in crop fields, wherein the crop plants are made resistant to HPPD chemistry by transformation to express genes encoding mutant HPPD polypeptides, and where an HPPD herbicide is applied as an over-the-top application in amounts capable of killing or impairing the growth of unwanted plant species (weed species, or, for example, carry-over or “rogue” or “volunteer” crop plants in a field of desirable crop plants). The application may be pre- or post emergence of the crop plants or of the unwanted species, and may be combined with the application of other herbicides to which the crop is naturally tolerant, or to which it is resistant via expression of one or more other herbicide resistance transgenes. See, e.g., U.S. Patent Application Publication No. 2004/0058427 and PCT Publication No. WO 98/20144.

In another embodiment, the invention also relates to a method of protecting crop plants from herbicidal injury. In the cultivation of crop plants, especially on a commercial scale, correct crop rotation is crucially important for yield stability (the achievement of high yields of good quality over a long period) and for the economic success of an agronomic business. For example, across large areas of the main maize-growing regions of the USA (the “central corn belt”), soya is grown as the subsequent crop to maize in over 75% of cases. Selective weed control in maize crops is increasingly being carried out using HPPD inhibitor herbicides. Although that class of herbicides has excellent suitability for that purpose, it can result in agronomically unacceptable phytotoxic damage to the crop plants in subsequent crops (“carry-over” damage). For example, certain soya varieties are sensitive to even very small residues of such HPPD inhibitor herbicides. Accordingly, the herbicide resistant or tolerant plants of the invention are also useful for planting in a locus of any short term carry-over of herbicide from a previous application (e.g., by planting a transgenic plant of the invention in the year following application of an herbicide to reduce the risk of damage from soil residues of the herbicide).

EXAMPLES

The invention is now described with reference to the following Examples. These Examples are provided for the purpose of illustration only, and the invention is not limited to these Examples, but rather encompasses all variations which are evident as a result of the teachings provided herein.

Example 1

Cloning, expression and assay of Avena-derived HPPD SEQ ID NO: 1 and determination of k_(cat), K_(mHPP) and koff values versus various HPPD herbicides

A DNA sequence, codon-optimised for expression in E. coli, was synthesised by GeneArt (Regensburg, Germany) to encode an HPPD derived from Avena sativa (SEQ ID NO: 1), cloned into pET24a and expressed in E. coli BL21(DE3) with 50 μg/ml kanamycin selection as described in PCT Publication No. WO 02/46387. Overnight cultures grown at 30° C. were used to inoculate 3×1 litre LB in shake flasks at a ratio of 1:100. Cultures were grown at 37° C., 220 rpm, until an A^(1cm)600 nm of 0.6-0.8 was reached. The temperature was decreased to 15° C. and induced with 0.1 mM IPTG. Cultures were grown overnight, and cells harvested after 15 min centrifugation at 10,000 g. Cells were stored at −20° C. until extraction. A cell pellet from 3 litres of shake flask culture (˜12 g) was thawed in extraction buffer (50 mM Tris, 10 mM sodium ascorbate, 2 mM DTT, 2 mM AEBSF, 10 μM trypsin inhibitor, 1 mM EDTA, pH 7.66) at a ratio of 1 ml buffer: 1 g cell paste.

The extract was passed through the cell disrupter at 30,000 psi, and centrifuged at 50,000 g for 25 minutes at 4° C. The extract may be optionally buffer exchanged down Sephadex® G25. Supernatants were beaded in liquid nitrogen and stored at −80° C. Levels of HPPD expression were estimated by Western blot analysis and using purified Avena (1-10 ng) as standard. Extracts were diluted 1:6000 and 1-10 μl were loaded onto 12% SDS PAGE. In addition, expression was quantified by comparing induced and uninduced SDS PAGE with Coomassie® (Imperial Chemicals Industries, Ltd., London UK) staining. Gels were blotted onto PVDF membrane and Western blots carried out using rabbit anti-wheat HPPD (1:6600) serum as primary antibody and goat anti-rabbit FITC-linked antibodies (1:600) as secondary antibody. Detection of bands was carried out by scanning on a Fluorimager™ 595 (GE Healthcare Ltd, Buckinghamshire UK) and peak quantification was carried out by using ImageQuant™ (GE Healthcare Ltd, Buckinghamshire UK). Plasmid DNA was reisolated from all transformed strains and the DNA sequence across the coding region confirmed.

From Western blots, the expression level of SEQ ID NO: 1 polypeptide expressed in the E. coli extract was estimated to be about 10-14 mg/ml out of a total soluble protein concentration of 33.5 mg/ml.

The concentration of active HPPD in the extract was also more accurately estimated by active site titration. For example a range of volumes of extract (typically 0-20 μl) were added to 50 mM BisTrisPropane buffer at pH 7.0 and at 25° C. containing 25 mM Na ascorbate, 4 μg/ml bovine catalase and 3 nmoles of ¹⁴C-labelled Compound A (1.81 GBq/mmol), in a total final assay volume of 425 μl.

Compound A

The radiolabel protein binding reaction was quenched after 3 minutes by the addition of 100 μl of 1 mM ‘cold’ Structure A. Protein was exchanged into 50 mM BisTrisPropane buffer at pH 7.0 containing 0.1M KCl by rapid chromatography down a NAP5 G25 Sephadex® column (GE Healthcare Ltd, Buckinghamshire UK) and ¹⁴C bound to protein fractions measured in Optiphase scintillant using a Tri-Carb 2900TR scintillation counter (Perkin Elmer, Wellesley, Mass.). The HPPD binding site concentration in the extract was calculated from the titration as described in PCT Publication No. WO 02/46387 and was estimated as 94.9, 78.3, and 82.3 (average 85.2) μM in one extract and 47.2 μM in another example.

The K_(mHPP) and k_(cat) values of the expressed HPPD were estimated on the basis of assays carried out at 25° C. in solutions of 50 mM BisTrisPropane buffer at pH 7.0 containing 25 mM Na ascorbate, 4 μg/ml bovine catalase (Sigma, St. Louis, Mo.), and a range of concentrations (typically 0.5×-10×K_(m)) of 4-hydroxyphenylpyruvate. Typically assays in a final volume of 110 μl were started with the addition of enzyme and accurately stopped after 20 or preferably 10 or 15 seconds with whirlimixed addition of 20 μl 25% perchloric acid. The assay solution was transferred to Chromacol 03-CVG HPLC vials, sealed and the amount of homogentisate formed in a 40 μl aliquot determined by injection onto a reverse phase Aqua C18 5μ 75×4.6 mm HPLC column running 5.5% acetonitrile 0.1% TFA (Buffer A) at 1.5 ml/min. The column was eluted at 1.5 ml/minute using a 2 minute wash in buffer A, followed by a 2 minute wash in a 30/70 mixture of buffer A and 100% acetonitrile, and a further 3.5 minute wash in buffer A. The elution of homogentisate was monitored by UV at 292 nm and the amount formed in each reaction quantified by comparison with a standard calibration curve.

K_(m) and V_(max) values were determined (for example FIG. 1) using a non linear least squares fit using Grafit 4™ software (Erithacus Software, Middlesex, UK). K_(cat) values were determined by dividing the maximum rate, V_(max) expressed in nmol/s by the number of nmoles of HPPD enzyme (based on the concentration determined by active-site titration).

From one set of separate experiments similar to those that produced the data shown in FIG. 1, on one extract of HPPD SEQ ID NO: 1 the K_(m) value was estimated as 6.17, 4.51, 6.09, 6.13, 4.37, 4.62, 5.41, 5.13 and 6 μM (K_(m) average=5.38 μM). The corresponding k_(cat) values were 4.92, 6.25, 7.08, 6.26, 5.5, 6.77, 6.89, 7.12 and 7.39 s⁻¹ (kcat average=6.46 s⁻¹). Note that for this calculation and, standardly herein, Mr was taken to be ˜94 kD and one active-site per dimer was assumed (i.e., half sites activity as well as inhibitor binding; see Garcia et al. (2000) Biochemistry, 39:7501-7507; Hawkes “Hydroxyphenylpyruvate Dioxygenase (HPPD)—The Herbicide Target.” In Modern Crop Protection Compounds. Eds. Krämer and Schirmer. Weinheim, Germany: Wiley-VCH, 2007. Ch. 4.2, pp. 211-220). For the alternative assumption of one active site per monomer then calculated k_(cat) values should simply be correspondingly and systematically halved.

On rates (governed by an association rate constant, k_(on)) for the formation of the enzyme:inhibitor complexes, EI and off rates (governed by a dissociation rate constant, k_(off)) were determined by methods known in the art and essentially as described in Hawkes et al. (2001) Proc. Bright. Crop. Prot. Conf. Weeds, 2:563-568 and in PCT Publication No. WO 02/46387).

For example, on rates were measured by, at zero time, adding ˜60 pmoles HPPD to 50 mM BisTrisPropane buffer at pH 7.0 and at 25° C. containing 25 mM Na ascorbate, 4 μg/ml bovine catalase (Sigma, St. Louis, Mo.) and an excess (˜300 pmoles) of ¹⁴C inhibitor in a total assay volume of 425 μl and, at various time points (0-180 s), quenching the radiolabel binding reaction by addition and rapid mixing of 100 μl ‘cold’ 1 mM structure A. Protein samples quenched at different times were then exchanged into 50 mM BisTrisPropane buffer at pH 7.0 containing 0.1M KCl by rapid chromatography down a NAP5 G25 Sephadex® column (GE Healthcare Ltd, Buckinghamshire UK) and the amount of ¹⁴C bound to protein fractions quantified in Optiphase scintillant using a Tri-Carb® 2900TR scintillation counter (Perkin Elmer, Wellesley, Mass.). The data were fit according to the scheme below in order to derive the value of the apparent second order rate constant, k2, governing the association rate of enzyme and radiolabelled inhibitor. A range of enzyme and inhibitor concentrations were used. Optionally, the rate constant may be derived from similar experiments where enzyme (at ˜0.05-0.2 μM binding sites) and, in this case, unlabelled, inhibitor (at ˜0.5 to 2 μM) are reacted for a range of short times (0-60s) in 50 mM BisTrisPropane buffer at pH7.0 and at 25° C. containing 25 mM Na ascorbate, 4 μg/ml bovine catalase (Sigma, St. Louis, Mo.) and then quenched by rapid dilution into assay solution containing 100-200 μM HPP for immediate assay by HPLC/UV quantitation of homogentisate formation after 30-40s (i.e., a time sufficiently short that inhibitor dissociation and association does not significantly occur on the timescale of the assay) as described above. Further example methods are described in PCT Publication No. WO 02/46387.

Off rates (k1 in the scheme below) were derived from exchange rate studies where either the test inhibitor, I, or its exchange partner, J were radiolabelled and the data fit according to the scheme below. As noted in Hawkes et al. (2001) Proc. Bright. Crop. Prot. Conf. Weeds, 2:563-568, HPPD preparations typically appear, at least in respect of the dissociation of certain inhibitors, to comprise 5-40% of a more rapidly exchanging (weaker binding) fraction of inhibitor binding sites. A small amount of this effect is due to non-specific binding, which can easily be subtracted or allowed for. For most inhibitors, the effect is small; with respect to structures like Compound A, such an effect is likely due to pro-chirality. Where fast and slow exchanging fractions are distinguished at all by inhibitors, here the measured off rates always refer only to the major slower exchanging fraction that represents 60-95% bulk of the HPPD inhibitor binding sites present in the extracts tested.

Off rates were determined by preincubating, for example, ˜200 pmoles of HPPD binding sites (determined as described above by active site titration in a 3 min reaction with structure A) in 50 mM BisTrisPropane buffer at pH 7.0 and at 25° C. containing 25 mM Na ascorbate, 4 μg/ml bovine catalase (Sigma, St. Louis, Mo.) containing ˜1.0 nmole ¹⁴C inhibitor at 25° C. in a total assay volume of 1.3 ml. After 30 minutes the exchange reaction was initiated with addition of 100 μl 1 mM ‘cold’ structure A with thorough mixing, and, immediately, 150 μl were withdrawn and loaded onto a NAP5 column, the protein exchanged into 50 mM BisTrisPropane buffer at pH 7.0 containing 0.1M KCl by rapid (<2 min) chromatography down a NAP5 G25 Sephadex® column (GE Healthcare Ltd, Buckinghamshire UK) and the amount of ¹⁴C bound to protein measured by Optiphase scintillant using a Tri-Carb® 2900TR scintillation counter (Perkin Elmer, Wellesley, Mass.). Further aliquots were removed and measured in the same way at various times over minutes or hours as required in order to determine the exchange kinetics.

In one variant of the method useful to better distinguish between off rates that were relatively rapid (e.g., where t½<15 min at 25° C.) the temperature of the experiment was reduced from 25° C. to ice temperature. In this case, off rates were determined by preincubating ˜200 pmoles HPPD in reaction buffer (50 mM BTP pH7, 25 mM Na ascorbate, 4 μg/ml bovine catalase, and 10% glycerol) containing ˜1.0 nmoles ¹⁴C inhibitor at 25° C. in a total assay volume of 1.3 ml. After 30 minutes the reaction vessel was transferred to ice. After a further 10 minutes at ice temperature the exchange reaction was initiated by addition of 100 μl 1 mM Structure A, with thorough mixing, and 150 μl was withdrawn, loaded and quickly exchanged down a NAP5 column in a cold room at ˜5-8° C. in order to quantify the amount of radiolabel remaining bound to the protein at various time from the start of exchange at ice temperature.

Off rates (k1) of HPPD inhibitors that are not readily available radiolabelled or that present other measurement problems (for example high levels of background non-specific protein-binding which can be measured as radiolabel binding that persists in the presence of high concentrations of ‘cold’ inhibitor) may be measured indirectly. In this case the enzyme complex (˜0.1-0.2 μM) is first formed with the unlabelled inhibitor and then the exchange kinetics derived by chasing it off with high a concentration of ¹⁴C-labelled structure A (or radiolabelled D) and monitoring the rate at which the label becomes bound to protein. Structure A is a particularly potent inhibitor with known kinetics. In a 20 fold or more excess Structure A will, in equilibrium, >90% occupy the binding sites in exchange competition with the other inhibitors tested here and indeed most other inhibitors (those skilled in the art will of course design the experiment/relative concentrations and fit the data accordingly). Exemplary methods are also described in PCT Publication No. WO 02/46387.

Exemplary on and off rate data (and derived K_(i) values) were obtained for the Avena-derived HPPD SEQ ID NO: 1 for the following compounds as follows.

Compound A (¹⁴C at 1.81 GBq/mmol)

Off rate k1=1.67E-05 s⁻¹ as determined at 25° C. using the direct, radiochemical method.

On rate k2=8.50E+04 M⁻¹ s⁻¹ as determined at 25° C. using the direct, radiochemical method.

K_(d)=1.96E-10 M.

K_(d)/K_(m) ratio=0.000036

Thus k_(off) was estimated as =1.67E-05 s⁻¹

Compound B (¹⁴C at 1.425 GBq/mmol)

Off rate k1(av)=8.1 E-04 s⁻¹ at 25° C. (individual experiments yielded k1=8.00E-04, 8.88E-04, 7.50E-04 and 8.00E-04 s⁻¹ as determined by the direct, radiochemical method).

Measured at ice temperature k1=1.58E-05 s⁻¹ (initial individual experiments yielded 1.16E-05 s⁻¹, 1.0E-05 s⁻¹, 1.2E-05 s⁻¹, 1.5E-05 s⁻¹. Later, more extensive experiments making better allowance for non-specific binding values came out consistently near and averaging around 1.58 E-05s⁻¹ (1.58E-05 s⁻¹, 1.5-05 s⁻¹, 1.67-05 s⁻¹, 1.5-05 s⁻¹. 1.58-05 s⁻¹. 1.58-05 s⁻¹, 1.5-05 s⁻¹).

On rate k2(av)=6.7E+04 s⁻¹ M⁻¹ at 25° C. (individual experiments yielded k2=6.35E+04, 7.50E+04, 6.2E+04 as determined by the direct radiochemical method). For mesotrione which has a relatively fast off rate estimates for on rate based on the activity-based method were more variable ranging from 4.2E+04 s⁻¹ M⁻¹, 4.9E+04 s⁻¹ M⁻¹ to 7.5 E+04 s⁻¹ M⁻¹ at 25° C.

K_(d) was thus estimated from the radiochemical data as 1.16E-08 M⁻¹ corresponding to a Kd/K_(m) ratio of 0.00217.

Thus k_(off) was estimated as 8.1 E-04 s⁻¹ at 25° C. and 1.58 E-05s⁻¹ at 0° C.

Compound C (¹⁴C at 0.774 GBq/mmol)

Off rate k1(av)=5.3 E-05 s⁻¹ at 25° C. (initial experiments yielded k1=7.80E-05 s⁻¹, 9.17E-05 s⁻¹, 4.5E-05 s⁻¹, 6E-05 s⁻¹, 7 E-05 s⁻¹ and 7.80E-05 s⁻¹; however a subsequent set of experiments making more accurate allowance for non specific binding effects makes it likely that the former high values were outliers). Based on later more consistent values, off rate k1(av)=5.3 E-05 s⁻¹ based on individual experimental values of 6.5-05 s⁻¹, 5.0-05 s⁻¹, 5.67-05 s⁻¹, 4.67-05 s⁻¹, 5.17-05 s⁻¹, 4.67-05 s⁻¹, 4.67-05 s⁻¹, and 6.0-05 s⁻¹.

The on rate k2, which was estimated to be 7.50E+03 s⁻¹ M⁻¹ at 25° C. using the direct radiochemical method is in good agreement with estimates from the enzyme activity-based method of 7.50E+03 s⁻¹ M⁻¹, 7.80E+03 s⁻¹ M⁻¹, 7.60E+03 s⁻¹ M⁻¹, 7.20E+03 s⁻¹ M⁻¹ and 1.0E+04 s⁻¹ M⁻¹ at 25° C.

Based on the radiochemical method the estimate of Kd=7.1 E-09M.

Therefore the estimate of Kd/Km ratio=0.0013.

Thus k_(off) was estimated as 5.3 E-05 s⁻¹.

Compound D (¹⁴C at 1.036 GBq/mmol)

Off rate k1=3.96E-05 s⁻¹ at 25° C. as determined using the direct, radiochemical method (individual measurements of 4.17E-05 s⁻¹ and 3.75E-05 s⁻¹). Off rate as determined in a series of indirect off rate experiments was 4.25-05 s⁻¹, 4.66-05 s⁻¹, 4.5-05 s⁻¹, 4.83-05 s⁻¹, 3.83-05 s⁻¹, 4.5-05 s⁻¹, 4.5-05 s⁻¹, 4.33-05 s⁻¹, 5.0-05 s⁻. Average value for off rate is taken to be 4.39E-05 s⁻¹ from all of the data.

On rate k2=3.20E+04 M⁻¹ s⁻¹ at 25° C. as determined by the direct radiochemical method. This is in fair agreement with estimates from the activity based method for on rate of 3.20E+04 M⁻¹ s⁻¹ and 5.7E+04 M⁻¹ s⁻¹.

Based on the radiochemical methods the estimate of Kd=1.36E-9 M.

The estimate of Kd/Km ratio=0.00025.

Thus k_(off) was estimated as 4.39E-05 s⁻¹.

Compound E

Off rate k1=4.17E-05 s⁻¹ at 25° C. as initially determined by the indirect, radiochemical method based on individual measurements of 5.50E-05 s⁻¹ and 2.85E-05 s⁻¹). From a series of further measurements the initial higher value would appear to be an outlier with values obtained of 3.67E-05 s⁻¹, 3.17E-05 s⁻¹, 2.67E-05 s⁻¹, 3.17E-05 s⁻¹, 2.67E-05 s⁻¹, 3.33E-05 s⁻¹, 3.08E-05 s⁻¹, 2.02E-05 s⁻¹, 3.00E-05 s⁻¹ and a new average koff value estimated as 2.96 E-05 s⁻¹.

On rate k2=1.30E+05 M⁻¹ s⁻¹ at 25° C. as determined by the direct non-radiochemical method.

The estimate of Kd=2.28E-10M.

The estimate of Kd/Km ratio=0.000042.

Thus k_(off) was estimated as 2.96 E-05 s⁻¹.

Compound F

From a series of indirect off rate measurements (7.08E-05 s⁻¹, 6.00E-05 s⁻¹, 7.00E-05 s⁻¹ and 6.83E-05 s⁻¹) the average k_(off) value was estimated as 6.72 E-05 s⁻¹.

Example 2

Cloning, expression and assay of HPPDs SEQ ID NOs: 1-14 from various plants and determination of k_(cat), Km_(HPP) and koff values versus various HPPD herbicides

DNA sequences optimized for E. coli codon usage and encoding HPPD polypeptides corresponding to SEQ ID NOs: 1-14 derived from various plants were synthesized by GeneArt (Regensburg, Germany), cloned into pET24a, and expressed in E. coli BL21(DE3) with 50 μg/ml kanamycin selection as described in PCT App. Pub. No. WO 02/46387. Cells were grown, protein extracts prepared, and HPPD active site titres and kinetic measurements (of k_(cat), K_(mHPP) and k_(off) values versus various different herbicides) carried out as described in Example 1.

The HPPD corresponding to SEQ ID NO: 1 was included as an internal control in experiments. The average absolute values of the various kinetic parameters for SEQ ID NO: 1 are listed above in detail in Example 1. The data in Table 2 below provide data from these measurements for SEQ ID NOs: 2-14 expressed as a ratio versus the corresponding control value for SEQ ID NO: 1. Thus all the parameter values for SEQ ID NO: 1 are given as 1.0 and all values in the table are comparative to those for SEQ ID NO: 1.

TABLE 2 Kinetic parameters of HPPDs of SEQ ID NOs: 2-14 (K_(cat)/K_(m))/ K_(off) B/ K_(off) C/ K_(off) D/ K_(off) E/ K_(off) F/ SEQ K_(cat)/K_(cat) K_(mHPP)/K_(m) (K_(cat)/K_(m)) K_(off) B K_(off) C K_(off) D K_(off) E K_(off) F ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID NO: NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2 2.0 3 1.1 1.1 0.9 2.1 1.3 1.3 1.5 1.6 1.3 1.2 1.1 2.2 1.1 1.2 1.5 4 1.3 1.6 0.8 0.5 0.6 1.6 1.3 1.6 0.8 5 1.5 1.4 1.1 0.5 0.7 1.5 1.7 1.8 0.9 6 1.0 1.1 0.9 1.0 1.8 1.0 7 1.1 0.9 1.2 1.1 2.1 1.7 1.3 0.9 0.8 1.1 1.3 1.7 1.7 8 1.1 0.8 1.3 1.3 1.4 1.1 1.2 0.8 1.4 9 1.2 1.1 1.1 0.1 0.2 0.5 10 1.4 <0.05 0.2 0.1 11 <0.1 0.3 0.3 0.3 12 0.1 0.1 0.2 0.2 13 0.7 0.7 1.0 <0.05 0.2 14 0.8 6.0 0.2 <0.05 4.0

‘k_(off) B’, ‘k_(off) C’, etc. refer to off rates measured by radiolabel exchange with the HPPD-inhibitor herbicides B-F described in Example 1.

It is apparent from the data in Table 2 that, relative to SEQ ID NO: 1, some HPPDs are more and others are less inherently resistant to the various herbicides B-F and therefore more or less suitable for conferring resistance in transgenic plants. In addition some exhibit greater or smaller values of k_(cat)/K_(m) relative to the HPPD of SEQ ID NO: 1. For each herbicide the relevant comparative parameter that determines the suitability or otherwise of the given HPPD sequence is the multiple of the k_(cat)/K_(m) (relative to that for SEQ ID NO: 1) value and the corresponding k_(off) rate for the herbicide (relative to that for SEQ ID NO: 1).

Thus, for example, the HPPD of SEQ ID NO: 3 derived from Alopecurus gives a value for this multiple of 2.3 for mesotrione (B) meaning that it is effectively about 2.3 fold more resistant to mesotrione than is SEQ ID NO: 1 and, all being equal, will confer a higher (probably about two fold) level of tolerance to mesotrione when likewise expressed in transgenic plants.

Similarly, the same sequence also provides enhanced (albeit to a lesser degree) tolerance over SEQ ID NO: 1 to the other herbicides tested (topramezone, tembotrione and the diketonitrile derived from isoxaflutole etc). Likewise it can be seen that the HPPDs of SEQ ID NOs: 6, 7 and 8 also confer advantages. On the other hand, of the range of other HPPDs tested, some appear significantly less effective than the HPPD of SEQ ID NO: 1 for providing tolerance to any herbicide. Thus, for example, the HPPDs from Erichola, Arabidopsis and Bidens appear to be of similar catalytic activity to the HPPD of SEQ ID NO: 1 but are >20 fold more sensitive to mesotrione whilst the Pseudomonas enzyme is both less catalytically efficient (k_(cat)/K_(m)˜0.2 of that for SEQ ID NO: 1) and more sensitive to mesotrione. Thus, a number of the new HPPD sequences described herein (e.g., SEQ ID NOs: 2-8) offer significant improvements over the prior art in respect of providing better options for providing tolerance to HPPD herbicides and especially in respect of the chemical classes exemplified.

Example 3

Cloning, expression and assay of mutant variants of HPPDs and determination of k_(cat), K_(mHPP) and k_(off) values versus various HPPD herbicides

DNA sequences optimized for E. coli codon usage and encoding HPPD polypeptides corresponding to SEQ ID NO: S:20-47, encoding HPPD polypeptides derived from Avena sativa, Alopecurus mysoides and Poa annua were synthesized by GeneArt (Regensburg, Germany), cloned into pET24a, and expressed in E. coli BL21(DE3) with 50 μg/ml kanamycin selection as described in PCT Publication No. WO 02/46387. Cells were grown, protein extracts were prepared, and HPPD active site titres and kinetic measurements of k_(cat), K_(mHPP), and k_(off) values were carried out as described in Example 1.

Within the present example, the following HPPD sequences derived from SEQ ID NO: 1 were used:

HPPD SEQ ID NO: 20 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358.

HPPD SEQ ID NO: 21 was changed relative to SEQ ID NO: 1 by the substitution of I for the V at position 217.

HPPD SEQ ID NO: 22 was changed relative to SEQ ID NO: 1 by the substitution of L for the V at position 217.

HPPD SEQ ID NO: 23 was changed relative to SEQ ID NO: 1 by the substitution of R for the A at position 326.

HPPD SEQ ID NO: 24 was changed relative to SEQ ID NO: 1 by the substitution of K for the A at position 326.

HPPD SEQ ID NO: 25 was changed relative to SEQ ID NO: 1 by the substitution of I for the A at position 326.

HPPD SEQ ID NO: 26 was changed relative to SEQ ID NO: 1 by the substitution of E for the I at position 339.

HPPD SEQ ID NO: 27 was changed relative to SEQ ID NO: 1 by the substitution of D for the I at position 339.

HPPD SEQ ID NO: 28 was changed relative to SEQ ID NO: 1 by the substitution of C for the I at position 339.

HPPD SEQ ID NO: 29 was changed relative to SEQ ID NO: 1 by the substitution of R for the G at position 408.

HPPD SEQ ID NO: 30 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358 and by the substitution of R for the A at position 326.

HPPD SEQ ID NO: 31 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358, by the substitution of R for the A at position 326, and by the substitution of I for the V at position 217.

HPPD SEQ ID NO: 32 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358 and by the substitution of I for the V at position 217.

HPPD SEQ ID NO: 33 was changed relative to SEQ ID NO: 1 by the substitution of R for the A at position 326 and by the substitution of I for the V at position 217.

HPPD SEQ ID NO: 34 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358, by the substitution of K for the A at position 326, and by the substitution of I for the Vat position 217.

HPPD SEQ ID NO: 35 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358, by the substitution of R for the A at position 326, and by the substitution of L for the V at position 217.

HPPD SEQ ID NO: 36 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358, by the substitution of R for the A at position 326, by the substitution of I for the V at position 217, and by the substitution of E for the I at position 339.

HPPD SEQ ID NO: 37 was changed relative to SEQ ID NO: 1 by the substitution of M for L at position 358, by the substitution of R for the A at position 326, by the substitution of L for the V at position 217, and by the substitution of E for the I at position 339.

HPPD SEQ ID NO: 42 was changed relative to SEQ ID NO: 1 by the substitution of H for the G at position 412.

HPPD SEQ ID NO: 43 was changed relative to SEQ ID NO: 1 by the substitution of C for the G at position 412.

HPPD SEQ ID NO: 44 was changed relative to SEQ ID NO: 1 by the substitution of A for the Q at position 297.

HPPD SEQ ID NO: 45 was changed relative to SEQ ID NO: 1 by the substitution of N for the Q at position 283.

HPPD SEQ ID NO: 46 was changed relative to SEQ ID NO: 1 by the substitution of G for the Q at position 297.

HPPD SEQ ID NO: 47 was changed relative to SEQ ID NO: 1 by the substitution of A for L at position 358.

Within the present example, the following HPPD sequences derived from SEQ ID NO: 3 were used:

HPPD SEQ ID NO: 40 was changed relative to SEQ ID NO: 3 by the substitution of M for L at position 359 and by the substitution of R for the A at position 327.

HPPD SEQ ID NO: 41 was changed relative to SEQ ID NO: 3 by the substitution of M for L at position 359, by the substitution of R for the A at position 327, and by the substitution of I for the V at position 218.

Within the present example, the following HPPD sequences derived from SEQ ID NO: 7 were used:

HPPD SEQ ID NO: 38 was changed relative to SEQ ID NO: 7 by the substitution of M for L at position 353 and by the substitution of R for the A at position 321.

HPPD SEQ ID NO: 39 was changed relative to SEQ ID NO: 7 by the substitution of M for L at position 353, by the substitution of R for the A at position 321, and by the substitution of I for the first V at position 213.

The HPPD corresponding to SEQ ID NO: 1 was included as an internal control in experiments. The average absolute values of the various kinetic parameters for the HPPD of SEQ ID NO: 1 are listed above in detail in Example 1. The data in Tables 3-5 below provide data from these measurements for SEQ ID NOs: 20-47 and 50 expressed as a ratio versus the corresponding control value for SEQ ID NO: 1. Thus all the parameter values for SEQ ID NO: 1 are given as 1.0.

TABLE 3 Kinetic parameters of HPPDs of SEQ ID NOs: 20-29 (K_(cat)/K_(m))/ K_(off) B/ K_(off) C/ K_(off) D/ K_(off) E/ K_(off) F/ SEQ K_(cat)/K_(cat) K_(mHPP)/K_(m) (K_(cat)/K_(m)) K_(off) B K_(off) C K_(off) D K_(off) E K_(off) F ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID NO: NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 20 0.7 0.9 0.8 2.0 1.1 2.3 1.2 2.1 0.8 0.9 0.8 1.9 1.9 21 1.1 1.4 0.8 1.5 1.1 1.0 1.0 1.1 0.9 0.9 1.0 1.5 22 0.8 0.8 1.0 2.0 1.0 1.2 1.0 1.0 0.7 0.7 1.1 23 1.5 1.0 1.6 1.7 1.4 1.2 1.3 1.3 0.6 2.2 1.7 0.9 1.4 1.3 24 0.8 1.0 0.8 1.4 1.1 1.5 1.2 1.3 0.7 0.6 1.2 25 0.9 1.6 1.1 1.3 26 0.6 0.5 1.2 1.5 1.0 1.4 1.6 1.5 0.6 0.6 1.0 27 0.8 0.9 0.9 1.7 1.1 1.5 0.7 0.7 1.1 28 1.2 1.0 1.6 29 0.1 0.6 0.2 8.1 1.7 11.3 0.1 0.3 0.4

TABLE 4 Kinetic parameters of HPPDs of SEQ ID NOs: 30-36 (K_(cat)/K_(m))/ K_(off) B/ K_(off) C/ K_(off) D/ K_(off) E/ K_(off) F/ SEQ K_(cat)/K_(cat) K_(mHPP)/K_(m) (K_(cat)/K_(m)) K_(off) B K_(off) C K_(off) D K_(off) E K_(off) F ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID NO: NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 30 0.6 0.6 1.1 3.4 2.8 1.6 3.2 0.6 0.5 1.1 3.3 2.7 1.6 3.1 0.8 0.8 1.1 3.5 1.1 3.1 1.9 0.8 0.7 1.1 3.6 0.9 2.8 2.2 0.8 0.7 1.0 31 0.6 0.5 1.1 5.3 1.1 3.2 2.4 3.8 0.6 0.5 1.1 5.3 2.6 2.3 0.8 0.7 1.2 32 1.0 1.1 0.8 3.1 1.0 2.1 1.6 2.7 0.9 0.9 0.9 3.2 2.3 33 0.6 0.5 0.8 2.5 1.3 1.4 1.5 1.3 0.6 0.7 0.7 34 0.7 0.8 0.9 5.0 3.2 2.9 0.7 0.9 0.8 35 0.8 1.2 0.7 6.3 3.0 2.5 0.7 0.8 0.9 36 0.7 0.7 1.0 8.5 3.6 5.1 0.7 0.5 1.3

TABLE 5 Kinetic parameters of HPPDs of SEQ ID NOs: 37-47 and 50 (K_(cat)/K_(m))/ K_(off) B/ K_(off) C/ K_(off) D/ K_(off) E/ K_(off) F/ SEQ K_(cat)/K_(cat) K_(mHPP)/K_(m) (K_(cat)/K_(m)) K_(off) B K_(off) C K_(off) D K_(off) E K_(off) F ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID SEQ ID NO: NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 NO: 1 1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 37 0.7 0.5 1.3 8.6 3.4 3.7 0.7 0.4 1.6 38 0.5 0.8 0.7 3.9 3.1 2.8 0.6 0.5 1.2 39 0.8 0.7 1.2 6.4 4.3 4.8 0.9 0.8 1.0 40 0.6 0.5 1.0 8.4 2.1 2.5 0.8 0.6 1.3 41 0.9 0.7 1.2 12.5 3.6 5.3 0.9 1.0 1.0 42 2.0 8.1 0.2 1.4 0.7 0.6 1.0 1.9 1.1 4.6 0.2 43 0.7 10.5 <0.1 2.0 1.4 1.0 2.7 0.7 0.5 8.5 <0.1 44 0.4 22 <0.1 1.6 0.6 45 <0.05 >2 <<0.1 2.0 46 0.1 49 <<0.1 2.7 1.8 47 0.2 2 0.1 1.0 1.7 50 1.1 2 1.1 15.3 3.2 4.7 3.4 3.7 4.1

It is apparent from the data depicted in Tables 3-5 that, relative to SEQ ID NO: 1, some mutant HPPDs are more and others are less inherently resistant to the various herbicides B-F and therefore more or less suitable for conferring resistance in transgenic plants. Again, for each herbicide the relevant comparative parameter that determines the suitability or otherwise of the given HPPD sequence is the multiple of the k_(cat)/K_(m) (relative to that for SEQ ID NO: 1) value and the corresponding k_(off) rate for the herbicide (relative to that for SEQ ID NO: 1). Thus, those sequences, for example SEQ ID NOs: 20-29, having single amino acid changes relative to SEQ ID NO: 1 at positions L358, V217, I339 and A326 of SEQ ID NO: 1 not only show significant increases in off rate (k_(off)) to one or more of the test herbicides (B-F) but also remain catalytically efficient (i.e. k_(cat)/K_(m) remains about the same or is improved over that for SEQ ID NO: 1). Thus, for example, the HPPD of SEQ ID NO: 26, which has an I339E amino acid substitution relative to SEQ ID NO: 1 exhibits a significant (˜1.5 fold) improvement in inherent tolerance to herbicides B, D, E and F relative to SEQ ID NO: 1. Similarly, the various other single amino acid substitutions (L358M, V217I, V217L, G408R, I339D, I339C, A326R, A326I and A326K of SEQ ID NOs: 20-29) also provided improvements in inherent tolerance to one or more of the herbicides.

SEQ ID NO: 29, with a G408R change relative to SEQ ID NO: 1, exhibits only about a quarter to a third of the catalytic activity of the HPPD of SEQ ID NO: 1. However this shortfall in catalytic activity is more than made up by the 8× or more increase in the value of k_(off) governing the rate of dissociation of compounds B and E from the enzyme. Further evidence demonstrates that mutating G408 confers advantageous properties on the mutant enzyme. For example, a G408A substitution has similar catalytic activity to the HPPD of SEQ ID NO: 1 (k_(cat)/K_(m)) and exhibits, compared to SEQ ID NO: 1, a 1.45 fold increase in the rate of dissociation (k_(off)) of the enzyme:inhibitor complex with mesotrione. The G408A mutant HPPD also exhibits, relative to SEQ ID NO: 1, a greater than 1.5× increase in the value of k_(off) in respect of inhibitor D and a greater than 3.5× increase in the value of K_(off) in respect of inhibitor E. As such, commercial levels of tolerance to HPPD herbicides, and particularly to the structural classes exemplied herein, is more readily attained through the additional improvements conferred by these disclosed mutations.

In order to provide any degree of useful tolerance, a mutation must provide an decrease in herbicide binding (here measured as an increase in the rate of dissociation of herbicide from the enzyme) that, in numerical magnitude, more than outweighs any catalytic deficit (here measured as k_(cat)/K_(m)). Even more valuable are those mutations that can be combined together with each other and incorporated into a variety of HPPD sequences (for example, the HPPDs set forth in Table 2) so as to work in combination and provide additive, or more preferably, synergistic levels of herbicide tolerance relative to native HPPDs.

Thus, for example, the variant HPPD of SEQ ID NO: 30 derived from Avena which combines the the A326R substitution of SEQ ID NO: 23 with the L358M substitution of SEQ ID NO: 20 provides a level of HPPD herbicide resistance that exceeds that of either the two single mutations and, indeed, provides a near multiplicative increase. So for example the HPPD of SEQ ID NO: 30 is about 3.5 fold more resistant to mesotrione than is SEQ ID NO: 1 and thus, all things being equal, will confer a higher (probably about three or more fold) level of tolerance to mesotrione when likewise expressed in transgenic plants. The same HPPD sequence (i.e., SEQ ID NO: 30) also provides enhanced tolerance over SEQ ID NO: 1 in respect of the other herbicides tested and, notably, for example, is about 3 fold more tolerant to topramezone and isoxaflutole (diketonitrile) than is SEQ ID NO: 1. The HPPD of SEQ ID NO: 31, which combines the V217I substitution of SEQ ID NO: 21 with the L358M substitution of SEQ ID NO: 20 and the A326R substitution of SEQ ID NO: 23 correspondingly shows even yet further (˜5.8 fold) enhancement in resistance to mesotrione and other herbicides over the HPPD of SEQ ID NO: 1. Similarly, the HPPD of SEQ ID NO: 36 which yet further adds the I339E change of SEQ ID NO: 26 to those of SEQ ID NO: 31 is yet even more tolerant to mesotrione (˜9.8 fold) than is the HPPD of SEQ ID NO: 1. This demonstrates that the changes at all of these amino acid positions can be combined to provide enhanced levels of inherent enzyme herbicide tolerance.

In addition the same mutational changes also function and beneficially alter the characteristics of HPPDs other than the SEQ ID NO: 1 from Avena. Thus, for example, SEQ ID NO: 40 combines the equivalent amino acid changes as in SEQ ID NO: 30 but this time in the Alopecurus HPPD of SEQ ID NO: 3. The benefits of the two mutational changes and of the Alopecurus HPPD enzyme over Avena work about multiplicatively so that, taking into account the values of k_(cat)/K_(m) and of k_(off), the HPPD of SEQ ID NO: 40 exhibits a more than 10 fold improvement in inherent tolerance to mesotrione relative to SEQ ID NO: 1. Similarly, SEQ ID NO: 39 combines the equivalent amino acid changes as in SEQ ID NO: 31 but this time into the Poa HPPD of SEQ ID NO: 7. Accordingly, SEQ ID NO: 39 exhibits significantly better tolerance to compound D (˜4.7 fold better than the HPPD of SEQ ID NO: 1) than does the HPPD of SEQ ID NO: 31 (˜3.2 fold better than the HPPD of SEQ ID NO: 1). SEQ ID NO: 41 also combines the equivalent amino acid changes as in SEQ ID NO: 31 but this time into the Alopecurus HPPD of SEQ ID NO: 3. The HPPD of SEQ ID NO: 41 exhibits a high level of inherent tolerance to both compound E (˜5.8 fold higher than the HPPD of SEQ ID NO: 1) and also to mesotrione (˜13.8 fold higher than the HPPD of SEQ ID NO: 1).

Example 4 Preparation and Evaluation of Herbicide Tolerance Conferred by Heterologous HPPD Enzymes Expressed in Tobacco

In the present example, native and mutant HPPDs are, for example, SEQ ID NOs: 1-14 and 20-47. DNA sequences that encode these (optimized for tobacco or, optionally, codon optimized according to a target crop such as soybean) are prepared synthetically and obtained commercially from GeneArt (Regensburg, Germany). Each sequence is designed to have 5′ Nde1 and 3′BamH1 sites to facilitate direct cloning and then cloned into a suitable binary vector for Agrobacterium-based plant transformation.

In a particular embodiment, tobacco-optimized genes encoding N-terminal fusions of the petunia EPSPS chloroplast transit peptide (SEQ ID NO: 48) to HPPD SEQ ID NOs: 13 and 14 and the wheat HPPD (SEQ ID NO: 5 in PCT Publication No. WO 02/46387; UNIPROT:A7WK82) are cloned into expression constructs and transformed into tobacco. The CTP/HPPD encoding nucleotide sequence is edited by PCR (or initially synthesized) to include a 5′ XhoI site, a TMV omega enhancer (SEQ ID NO: 86) and a 3′ KpnI site (and to remove any such internal sites).

The expression cassette, comprising the TMV omega 5′ leader, CTP and 4-HPPD gene is excised using XhoI/KpnI and cloned into similarly digested pBIN 19 (Bevan, Nucl. Acids Res. (1984) behind a double enhanced 35S promoter (SEQ ID NO: 87) and ahead of a NOS 3′ transcription terminator (SEQ ID NO: 88) and then transformed into E. coli TOP 10 competent cells. DNA recovered from the E. coli is used to transform Agrobacterium tumefaciens LBA4404, and transformed bacteria are selected on media contain rifampicin and kanamycin. Tobacco tissue is subjected to Agrobacterium-mediated transformation using methods known in the art or as described herein. For example, a master plate of Agrobacterium tumefaciens containing the HPPD expressing binary vector is used to inoculate 10 ml LB (L broth) containing 100 mg/l rifampicin plus 50 mg/l kanamycin using a single bacterial colony. This is incubated overnight at 28° C. shaking at 200 rpm. This entire overnight culture is used to inoculate a 50 ml volume of LB containing the same antibiotics. Again this is cultured overnight at 28° C. shaking at 200 rpm. The Agrobacterium cells are pelleted by centrifuging at 3000 rpm for 15 minutes and then resuspended in MS (Murashige and Skoog) medium containing 30 g/l sucrose, pH 5.9 to an OD (600 nM)=0.6. This suspension is dispensed in 25 ml aliquots into petri dishes.

Clonally micro-propagated tobacco shoot cultures are used to excise young (not yet fully expanded) leaves. The mid rib and outer leaf margins are removed and discarded, and the remaining lamina cut into 1 cm squares. These are transferred to the Agrobacterium suspension for 20 minutes. Explants are then removed, dabbed on sterile filter paper to remove excess suspension, then transferred onto solid NBM medium (MS medium containing 30 g/l sucrose, 1 mg/l BAP (benzylaminopurine) and 0.1 mg/l NAA (napthalene acetic acid) at pH 5.9 and solidified with 8 g/l plantagar), with the abaxial surface of each explant in contact with the medium. Approximately 7 explants are transferred per plate, which are then sealed and maintained in a lit incubator at 25° C. for a 16 hour photoperiod for 3 days.

Explants are then transferred onto NBM medium containing 100 mg/l kanamycin plus antibiotics to prevent further growth of Agrobacterium (200 mg/l timentin with 250 mg/l carbenicillin). Further subculture onto this same medium was then performed every 2 weeks.

As shoots start to regenerate from the callusing leaf explants, these are removed to Shoot elongation medium (MS medium, 30 g/l sucrose, 8 g/l plantagar, 100 mg/l kanamycin, 200 mg/l timentin, 250 mg/l carbenicillin, pH 5.9). Stable transgenic plants readily root within 2 weeks. To provide multiple plants per event to ultimately allow more than one herbicide test per transgenic plant, all rooting shoots are micropropagated to generate 3 or more rooted clones.

Putative transgenic plants that are rooting and showing vigorous shoot growth on the medium incorporating kanamycin are analysed by PCR using primers that amplified a 500 bp fragment within the HPPD transgene. Evaluation of this same primer set on untransformed tobacco showed conclusively that these primers would not amplify sequences from the native tobacco HPPD gene.

Transformed shoots are divided into 2 or 3 clones and regenerated from kanamycin resistant callus. Shoots are rooted on MS agar containing kanamycin. Surviving rooted explants are re-rooted to provide approximately 40-50 kanamycin resistant and PCR positive events from each event.

Once rooted, plantlets are transferred from agar and potted into 50% peat, 50% John Innes Soil No. 3 with slow-release fertilizer in 3 inch round pots and left regularly watered to establish for 8-12 days in the glass house. Glass house conditions were adjusted to about 24-27° C. day, 18-21° C. night and approximately a 14 hour photoperiod. Humidity was adjusted to ˜65% and light levels were up to 2000 μmol/m² at bench level.

Three transgenic populations each of about forty tobacco plants and comprising, alternatively, an HPPD gene encoding the wheat HPPD (A7WK82), HPPD SEQ ID NO: 12 or HPPD SEQ ID NO: 13 were thus produced. A sub-set of about 30 plants were then selected on the basis of similar size from each population for spray testing. All the plants were then sprayed with 600 g/ha of mesotrione. Callisto® was mixed in water with 0.2-0.25% X-77® surfactant and sprayed from a boom on a suitable track sprayer moving at 2 mph with the nozzle about 2 inches from the plant tops. Spray volume was 200 l/ha. Plants were assessed for damage and scored at 14 days after treatment (DAT). The results are depicted in Table 6.

The black boxes in Table 6 indicate that the plants in that particular event were green, while the gray boxes indicate that the plants were partly bleached. The remaining white boxes indicate that the plants were fully bleached. From the data depicted in Table 6 it is clear that, of the like-expressed HPPDs, only the wheat HPPD(A7WK82) had conferred a useful level of tolerance to mesotrione (i.e., normally robust to at least 2-4× of a normal field application rate). In contrast, the HPPDs of SEQ ID NO: 13 (from Arabidopsis) and SEQ ID NO: 14 (from Pseudomonas fluorescens) provided no effective tolerance at the rate sprayed. Five out of 33 wheat HPPD events exhibited < or equal to 10% damage (zero to slight stunting). This differential in the mesotrione tolerance according to which HPPD is expressed is entirely consistent both with the in vitro data depicted in Table 2 (indicating that neither the Arabidopsis nor Pseudomonas HPPDs have much inherent tolerance to mesotrione) and also with the published data describing the relative significant superiority of the wheat HPPD (both in vitro and in planta) to either of these (Hawkes et at (2001) in Proc. Brit Crop Prot. Conf. (Weeds), p 563. British Crop Protection Council and PCT Publication No. WO 02/46387).

TABLE 6

Table 7 depicts an assessment of damage 14 DAT with 600 g/ha isoxaflutole of clonal plants of the same events as depicted in Table 6 from an experiment carried out in parallel at the same time.

TABLE 7

All of the isoxaflutole-sprayed plants were damaged with only 3/33 wheat HPPD-expressing events showing less than 20% damage. Again, none of the Arabidopsis HPPD expressing plants appeared appreciably resistant whereas the Pseudomonas HPPD did confer some degree of resistance with 4/32 plants showing 50% damage or less. Again this is broadly consistent with the in vitro kinetic data (Table 2 and PCT Publication No. WO 02/46387), showing that while exhibiting a high K_(m) value for HPP and therefore exhibiting a relatively poor value of k_(at)/K_(m) the Pseudomonas HPPD does exhibit a relatively high value of k_(off) for the diketonitrile of isoxaflutole (i.e., Compound D).

The foregoing data broadly establish the predictive power of determining the in vitro enzyme kinetics, as determined by the relative value of (k_(cat)/K_(m))×k_(off), to anticipate how much tolerance a given HPPD will confer to a given HPPD herbicide when expressed in a crop plant.

In further examples, tobacco-optimized genes encoding HPPD SEQ ID NOs. 1 to 14 and HPPD SEQ NOs: 20-47 are cloned (this time without any CTP) into expression constructs, as described below, and transformed into tobacco. The HPPD encoding nucleotide sequence is edited by PCR (or initially synthesized) to include 5′ XhoI site, a TMV omega enhancer and a 3′ KpnI site (and to remove any such internal sites). The expression cassette, comprising the TMV omega leader and 4-HPPD gene is excised using XhoI/KpnI and cloned into similarly digested pBIN 19 (Bevan, Nucl. Acids Res. (1984) behind a double 35S promoter and ahead of a Nos gene transcriptional terminator and then transformed into E. coli TOP 10 competent cells. Again, transgenic populations of tobacco plants are generated and assessed in the glass-house as described above.

Example 5 Construction of Soybean Transformation Vectors

Binary vectors for dicot (soybean) transformation were constructed with a promoter, such as a synthetic promoter containing CaMV 35S and FMV transcriptional enhancers driving the expression of HPPD coding sequence, such as SEQ ID NOs: 1-8 and 20-41, followed by a Nos gene 3′ terminator. The HPPD gene was codon-optimized for soybean expression based upon the predicted amino acid sequence of the HPPD gene coding region. In the case that HPPD itself is not used as the selectable marker, Agrobacterium binary transformation vectors containing an HPPD expression cassette are constructed by adding a transformation selectable marker gene. For example, binary transformation vector 17900 (SEQ ID NO: 51) contains an expression cassette encoding an HPPD variant (SEQ ID NO: 49) linked with two PAT gene cassettes (one with the 35S promoter and one with the CMP promoter, and where both PAT genes are followed by the nos terminator) for glufosinate based selection during the transformation process. Another binary transformation vector (17901; SEQ ID NO: 52) contains an expression cassette encoding the HPPD variant (SEQ ID NO: 50) and also an EPSPS selectable marker cassette. Vector 17901 is transformed into soybean and transgenic plants are obtained using glyphosate selection using Agrobacterium-mediated transformation of immature seed targets. The DNA sequences encoding the HPPD genes are codon-optimized for expression in dicot plants.

The example binary vectors described above are constructed using a combination of methods well known to those skilled in the art such as overlap PCR, DNA synthesis, restriction fragment sub-cloning and ligation. Their unique structures are made explicit in FIG. 3 (vector 17900) and FIG. 4 (vector 17901) and in the sequence listings SEQ ID NOs: 51 and 52. Additional information regarding the vectors is provided below.

The abbreviations used in FIG. 3 (vector 17900) are defined as follows:

cAvHPPD-04

-   -   Start: 1024 End: 2343     -   Soybean codon optimized oat HPPD gene encoding SEQ ID NO: 49

cPAT-03-01

-   -   Start: 3209 End: 3760     -   PAT Hoescht A02774 synthetic S. viridochromogenes, plant codons;         identical to Q57146 phosphinothricin acetyl transferase protein

cPAT-03-02

-   -   Start: 5062 End: 5613     -   PAT Q57146 S. viridochromogenes phosphinothricin acetyl         transferase protein, cPAT-03-01 DNA, with mutated BamH1, Bgl2         sites

cSpec-03

-   -   Start: 6346 End: 7134     -   Also called aadA; gene encoding the enzyme aminoglycoside         3′adenyltransferase that confers resistance to spectinomycin and         streptomycin for maintenance of the vector in E. coli and         Agrobacterium.

cVirG-01

-   -   Start: 7434 End: 8159     -   virG (putative) from pAD1289 with TTG start codon. virGN54D came         from pAD1289 described in Hansen et al. 1994, PNAS 91:7603-7607

cRepA-01

-   -   Start: 8189 End: 9262     -   RepA, pVS1 replication protein

eNOS-01

-   -   Start: 168 End: 259     -   Putative NOS enhancer sequence from 15235 as found in the right         border of certain binary vectors.

eFMV-03

-   -   Start: 396 End: 589     -   Enhancer region from figwort mosaic virus (FMV)

e35S-05

-   -   Start: 596 End: 888     -   C to T & C to A by changes in CMV 35S enhancer region

eTMV-02

-   -   Start: 953 End: 1020     -   TMV Omega 5′UTR leader seq thought to enhance expression. EMBL:         TOTMV6

eFMV-03

-   -   Start: 4054 End: 4247     -   Enhancer region from figwort mosaic virus (FMV)

e35S-05

-   -   Start: 4254 End: 4546     -   C to T & C to A by changes in CMV 35S enhancer region

eNOS-01

-   -   Start: 4557 End: 4648     -   Putative NOS enhancer sequence from 15235 as found in the right         border of certain binary vectors.

bNRB-05

-   -   Start: 4 End: 259 (complementary)     -   Right border/NOS T-DNA region; may influence promoters. EMBL         Nos: J01826, V00087, AF485783.

bNRB-01-01

-   -   Start: 101 End: 125 (complementary)     -   Right Border Repeat of T-DNA of Agrobacterium tumefaciens         nopaline ti-plasmid

bNLB-03

-   -   Start: 5937 End: 6066 (complementary)     -   Left border region of T-DNA of Agrobacterium tumefaciens         nopaline ti-plasmid     -   Start: 5972 End: 5996 (complementary)     -   25 bp Left border repeat region of T-DNA of Agrobacterium         tumefaciens nopaline ti-plasmid

prCMP-04

-   -   Start: 4655 End: 5051     -   Cestrum yellow leaf curl virus promoter & leader. Genbank®         Accession No. AF364175. See also U.S. Patent Application         Publication No. 20040086447. prCMP-01 with 1 base pair         truncation on 5′ end and 2 base pair truncation on 3′ end

pr35S-04-01

-   -   Start: 2664 End: 3184     -   35S promoter from CMV. EMBL: CAMVG2

oVS1-02

-   -   Start: 9305 End: 9709     -   Origin of replication and partitioning region from plasmid pVS1         of Pseudomonas (Itoh et al. 1984, Plasmid 11: 206-220); similar         to GenBank® Accession Number U10487; serves as origin of         replication in Agrobacterium tumefaciens host

oCOLE-06

-   -   Start: 10387 End: 11193 (complementary)     -   The ColE1 origin of replication functional in E. coli derived         from pUC19

tNOS-05-01

-   -   Start: 2360 End: 2612     -   Synthetic nopaline synthetase terminator

tNOS-05-01

-   -   Start: 3794 End: 4046     -   Synthetic nopaline synthetase terminator     -   tNOS-05-01     -   Start: 5642 End: 5894     -   Synthetic nopaline synthetase terminator

The abbreviations used in FIG. 4 (vector 17901) are defined as follows:

cAmHPPD-01

Start: 1024 End: 2346

tobacco codon optimized Alopecurus mycosuroides HPPD gene encoding SEQ ID NO: 50

cGmEPSPS-01

Start: 3675 End: 5252

Soybean codon-optimized version of double mutant soybean EPSPS cDNA

cSpec-03

-   -   Start: 6346 End: 7134     -   Also called aadA; gene encoding the enzyme aminoglycoside         3′adenyltransferase that confers resistance to spectinomycin and         streptomycin for maintenance of the vector in E. coli and         Agrobacterium.

cVirG-01

-   -   Start: 7434 End: 8159     -   virG (putative) from pAD1289 with TTG start codon. virGN54D came         from pAD1289 described in Hansen et al. 1994, PNAS 91:7603-7607

cRepA-01

-   -   Start: 8189 End: 9262     -   RepA, pVS1 replication protein

eNOS-01

-   -   Start: 168 End: 259     -   Putative NOS enhancer sequence from 15235 as found in the right         border of certain binary vectors.

eFMV-03

-   -   Start: 396 End: 589     -   Enhancer region from figwort mosaic virus (FMV)

e35S-05

-   -   Start: 596 End: 888     -   C to T & C to A by changes in CMV 35S enhancer region

eTMV-02

-   -   Start: 953 End: 1020     -   TMV Omega 5′UTR leader seq thought to enhance expression. EMBL:         TOTMV6

eFMV-03

-   -   Start: 4054 End: 4247     -   Enhancer region from figwort mosaic virus (FMV)

e35S-05

-   -   Start: 4254 End: 4546     -   C to T & C to A by changes in CMV 35S enhancer region

eNOS-01

-   -   Start: 4557 End: 4648     -   Putative NOS enhancer sequence from 15235 as found in the right         border of certain binary vectors.

bNRB-05

-   -   Start: 4 End: 259 (complementary)     -   Right border/NOS T-DNA region; may influence promoters. EMBL         Nos: J01826, V00087, AF485783.

bNRB-01-01

-   -   Start: 101 End: 125 (complementary)     -   Right Border Repeat of T-DNA of Agrobacterium tumefaciens         nopaline ti-plasmid

bNLB-03

-   -   Start: 5937 End: 6066 (complementary)     -   Left border region of T-DNA of Agrobacterium tumefaciens         nopaline ti-plasmid     -   Start: 5972 End: 5996 (complementary)     -   25 bp Left border repeat region of T-DNA of Agrobacterium         tumefaciens nopaline ti-plasmid

prCMP-04

-   -   Start: 4655 End: 5051     -   Cestrum yellow leaf curl virus promoter & leader. Genbank®         Accession No. AF364175. See also U.S. Patent Application         Publication No. 20040086447. prCMP-01 with 1 base pair         truncation on 5′ end and 2 base pair truncation on 3′ end

oVS1-02

-   -   Start: 9305 End: 9709     -   Origin of replication and partitioning region from plasmid pVS1         of Pseudomonas (Itoh et al. 1984, Plasmid 11: 206-220); similar         to GenBank® Accession Number U10487; serves as origin of         replication in Agrobacterium tumefaciens host

oCOLE-06

-   -   Start: 10387 End: 11193 (complementary)     -   The ColE1 origin of replication functional in E. coli derived         from pUC19

tNOS-05-01

-   -   Start: 2360 End: 2612     -   Synthetic nopaline synthetase terminator

tNOS-05-01

-   -   Start: 3794 End: 4046     -   Synthetic nopaline synthetase terminator

Example 6 Transformation of Soybean and Selection of Herbicide-Resistant Plants

Soybean plant material can be suitably transformed and fertile plants regenerated by many methods which are well known to one of skill in the art. For example, fertile morphologically normal transgenic soybean plants may be obtained by: 1) production of somatic embryogenic tissue from, e.g., immature cotyledon, hypocotyl or other suitable tissue; 2) transformation by particle bombardment or infection with Agrobacterium; and 3) regeneration of plants. In one example, as described in U.S. Pat. No. 5,024,944, cotyledon tissue is excised from immature embryos of soybean, preferably with the embryonic axis removed, and cultured on hormone-containing medium so as to form somatic embryogenic plant material. This material is transformed using, for example, direct DNA methods, DNA coated microprojectile bombardment or infection with Agrobacterium, cultured on a suitable selection medium and regenerated, optionally also in the continued presence of selecting agent, into fertile transgenic soybean plants. Selection agents may be antibiotics such as kanamycin, hygromycin, or herbicides such as phosphonothricin or glyphosate or, alternatively, selection may be based upon expression of a visualisable marker gene such as GUS. Alternatively, target tissues for transformation comprise meristematic rather than somaclonal embryogenic tissue or, optionally, is flower or flower-forming tissue. Other examples of soybean transforamtions can be found, e.g. by physical DNA delivery method, such as particle bombardment (Finer and McMullen (1991) In Vitro Cell Dev. Biol. 27P:175-182; McCabe et al. (1988) Bio/technology 6:923-926), whisker (Khalafalla et al. (2006) African J. of Biotechnology 5:1594-1599), aerosol bean injection (U.S. Pat. No. 7,001,754), or by Agrobacterium-mediated delivery methods (Hinchee et al. (1988) Bio/Technology 6:915-922; U.S. Pat. No. 7,002,058; U.S. Patent Application Publication No. 20040034889; U.S. Patent Application Publication No. 20080229447; Paz et al. (2006) Plant Cell Report 25:206-213). The HPPD gene can also be delivered into organelle such as plastid to confer increased herbicide resistance (see U.S. Patent Application Publication No. 20070039075).

Soybean transgenic plants can be generated with the heretofore described binary vectors containing HPPD gene variants with different transformation methods. Optionally, the HPPD gene can provide the means of selection and identification of transgenic tissue. For example, a vector was used to transform immature seed targets as described (see e.g., U.S. Patent Application Publication No. 20080229447) to generate transgenic HPPD soybean plants directly using HPPD inhibitor, such as mesotrione, as selection agent. Optionally, HPPD genes can be present in the polynucleotide alongside other sequences which provide additional means of selection/identification of transformed tissue including, for example, the known genes which provide resistance to kanamycin, hygromycin, phosphinothricin, butafenacil, or glyphosate. For example, different binary vectors containing PAT or EPSPS selectable marker genes as described in Example 4 were transformed into immature soybean seed target to generate HPPD herbicide tolerant plants using Agrobacterium-mediated transformation and glufosinate or glyphosate selection as described (see e.g., U.S. Patent Application Publication No. 20080229447).

Alternatively selectable marker sequences may be present on separate polynucleotides and a process of, for example, co-transformation and co-selection is used. Alternatively, rather than a selectable marker gene, a scorable marker gene such as GUS may be used to identify transformed tissue.

An Agrobacterium-based method for soybean transformation can be used to generate transgenic plants using glufosinate, glyphosate or HPPD inhibitor mesotrione as selection agent using immature soybean seeds as described (U.S. Patent Application Publication No. 20080229447).

Example 7 Soybean T₀ Transgenic Plant Growth, Analysis and Herbicide Tolerance Evaluation

T₀ plants were taken from tissue culture to the greenhouse where they were transplanted into water-saturated soil (Redi-Earth® Plug and Seedling Mix, Sun Gro Horticulture, Bellevue, Wash.) mixed with 1% granular Marathon® (Olympic Horticultural Products, Co., Mainland, Pa.) at 5-10 g/gal Redi-Earth® Mix in 2″ square pots. The plants were covered with humidity domes and placed in a Conviron chamber (Pembina, N. Dak.) with the following environmental conditions: 24° C. day; 18° C. night; 16 hr light-8 hrs dark photoperiod; and 80% relative humidity.

After plants become established in the soil and new growth appears (˜1-2 weeks), plants are sampled and tested for the presence of desired transgene by Taqman™ analysis using appropriate probes for the HPPD genes, or promoters (for example prCMP and prUBq3). All positive plants and several negative plants are transplanted into 4″ square pots containing MetroMix® 380 soil (Sun Gro Horticulture, Bellevue, Wash.). Sierra 17-6-12 slow release fertilizer is incorporated into the soil at the recommended rate. The negative plants serve as controls for the spray experiment. The plants are then relocated into a standard greenhouse to acclimatize (˜1 week). The environmental conditions are typically: 27° C. day; 21° C. night; 16 hr photoperiod (with ambient light); ambient humidity. After acclimatizing (˜1 week), the plants are ready to be sprayed with the desired herbicides. Herbicide tolerant transgenic soybean plants are grown to maturity for seed production. Transgenic seeds and progeny plants are used to further evaluate their herbicide tolerance performance and molecular characteristics.

Thus T1 soybean plants of vector 17900 (FIG. 3) and of vector 17901 (FIG. 4) expressing, alternatively, SEQ ID NO: 49 and SEQ ID NO: 50 from identical expression cassettes are grown and tested for tolerance to a range of HPPD herbicides in comparison with like plants, likewise expressing HPPD SEQ ID NO: 1.

All patents, patent applications and publications mentioned in the specification are indicative of the level of those skilled in the art to which this invention pertains. All patents, patent applications and publications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.

While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention can be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims include all such embodiments and equivalent variations. 

1. An expression cassette comprising a polynucleotide encoding a polypeptide operably linked to a heterologous promoter that drives expression in a plant or plant cell, wherein said polypeptide is plant-derived and has 4-hydroxyphenylpyruvate dioxygenase (HPPD) enzymatic activity and comprises at least one of the following amino acid sequences: a) (L,I,R)(V,A)(G,A)DVL(S,T) (SEQ ID NO: 15), wherein the first L, the I, or the R is replaced with any other amino acid; b) G(I,V)LVD(R,K) SEQ ID NO: 16, wherein the L is replaced with any other amino acid; c) GGF(E,D)F(M,L)(A,P) (SEQ ID NO: 18), wherein the A or P is replaced with any other amino acid; d) CGGFGKGN (SEQ ID NO: 19), wherein the second G is replaced with any other amino acid; e) CGGFGKGN (SEQ ID NO: 19), wherein the K is replaced with any other amino acid.
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. (canceled)
 6. The expression cassette of claim 3, wherein the nucleotide sequence of the polynucleotide is optimized for expression in a plant.
 7. (canceled)
 8. The expression cassette of claim 3, further comprising an operably linked isolated polynucleotide sequence encoding a polypeptide that confers a desirable trait.
 9. The expression cassette of claim 8, wherein the desirable trait is resistance or tolerance to an herbicide.
 10. The expression cassette of claim 9, wherein said desirable trait is resistance or tolerance to an HPPD inhibitor, glyphosate, or glufosinate.
 11. The expression cassette of claim 10, wherein said polypeptide that confers a desirable trait is a cytochrome P450 or variant thereof.
 12. The expression cassette of claim 10, wherein said polypeptide that confers a desirable trait is an EPSPS (5-enol-pyrovyl-shikimate-3-phosphate-synthase).
 13. The expression cassette of claim 10, wherein said polypeptide that confers a desirable trait is a phosphinothricin acetyl transferase (PAT).
 14. A vector comprising the expression cassette of claim
 3. 15. (canceled)
 16. A method for conferring resistance or tolerance to an HPPD inhibitor in a plant, the method comprising introducing the expression cassette of claim 3 into the plant.
 17. A transformed plant cell comprising the expression cassette of claim
 3. 18. The transformed plant cell of claim 17, wherein the transformed plant cell is from a plant selected from the group consisting of rice, barley, potato, sweet potato, canola, sunflower, rye, oats, wheat, corn, soybean, sugar beet, tobacco, Miscanthus grass, Switch grass, safflower, trees, cotton, cassava, tomato, sorghum, alfalfa, sugar beet, and sugarcane.
 19. The transformed plant cell of claim 17, wherein the transformed plant cell is a soybean plant cell.
 20. A plant, plant part, or seed comprising the plant cell of claim
 17. 21. A method of controlling weeds at a locus comprising crop plants and weeds, wherein the crop plants comprise a plant according to claim 20, the method comprising applying to the locus a weed-controlling amount of one or more HPPD inhibitors.
 22. The method of claim 21, wherein the one or more HPPD inhibitors are selected from the group consisting of bicyclopyrone, benzobicyclon, benzofenap, ketospiradox or its free acid, isoxachlortole, isoxaflutole, mesotrione, pyrasulfotole, pyrazolynate, pyrazoxyfen, sulcotrione, tefuryltrione, tembotrione, topramezone, and agrochemically acceptable salts thereof.
 23. The method of claim 21, wherein the one or more HPPD inhibitors is mesotrione.
 24. The expression cassette of claim 3, wherein the first L, the I, or the R of SEQ ID NO: 15 is replaced with E, C, A or D.
 25. The expression cassette of claim 3, wherein the L of SEQ ID NO: 16 is replaced with M.
 26. The expression cassette of claim 3, wherein the A or P of SEQ ID NO: 18 is replaced with R, I, L, H or K.
 27. The expression cassette of claim 3, wherein the second G of SEQ ID NO: 19 is replaced with R, K, H, E, D, N, Q, A, S or T.
 28. The expression cassette of claim 3, wherein the K of SEQ ID NO: 19 is replaced with S or T.
 29. The expression cassette of claim 3, wherein the polypeptide additionally comprises the amino acid sequence DH(V, I, M)VGN (SEQ ID NO: 17), wherein the first V, the I or the M is replaced with L or I.
 30. The expression cassette of claim 3, wherein the polypeptide comprises at least two of the amino acid sequences found in sub-parts a-e.
 31. The expression cassette of claim 3, wherein the polypeptide comprises at least three of the amino acid sequences found in sub-parts a-e.
 32. The expression cassette of claim 3, wherein the polypeptide comprises all four of the amino acid sequences found in sub-parts a-e. 